Category (or Series) Romance novels are those small and inexpensive paperbacks with sweet and happy couples portrayed on the front cover.  You find them in stores like K-Mart and Target but seldom in bookshops. 
Famous historical novels like “Gone with the Wind” are not Category Romance novels.  The love story between Rhett and Scarlett is central to “Gone with the Wind”, but is not its only theme.  Category novels are much more narrowly focussed on the relationship between their protagonists.
Romance novels are about love relationships.  The tradition started by Jane Austen with her romantic novels set in the Regency Era (1811-1820) endures, but the modern romance novels have developed far beyond the intrigues and the rich dresses of British aristocracy of the early 1800s.  To convince yourself how true such a statement is, you only need to look at the web site of 
Mills & Boon/Harlequin, the best known publishers of Category Romance.  Harlequin is a Canadian publisher that acquired M&B (a UK publisher) some decades ago.
Before I talk about Romance Writing, let me give you some figures, which I got from Wikipedia.  In 2008, M&B sold 200 million novels per year, and is currently publishing about 100 books and 100 e-books per month. Harlequin is currently releasing 120 new titles each month in 29 different languages in 107 international markets.  In the UK alone, M&B has over 3 million regular readers.  If you thought that Romance novels were a niche market, think again!
I will reproduce here how M&B defines the characteristics of some of their series (i.e., imprints), extracted from the 
Australian submission guidelines.  They publish several books in each imprint every month.
Sexy: These stories are all about passion and escape, glamorous international settings, captivating women and the seductive, tempting men who want them. Length: 50,000 words.  Spine colour: red.
Sweet Romance: Sweet Romance stories are all about real, relatable women and strong, deeply desirable men experiencing the intensity, anticipation and sheer rush of falling in love.  Length: 50,000 words.  Spine colour: light blue.
Medical: Intense and uplifting romances set in the medical world. Experience the breath-taking rollercoaster of emotions, ambitions and desires of today's medical professionals. Length: 50,000 words.  Spine colour: teal.
Historical: Richly textured, emotionally intense novels set across a wide range of historical periods - ancient civilisations up to and including Second World War.  Length: 65,000 words.  Spine colour: blue.
Blaze: Blaze is Mills & Boon's sexiest romance series, yet there's more to these books than simply sex. We ask our authors to deliver complex plots and subplots, realistic engaging characters and a consuming love story you won't be able to forget. Blaze stories are fun, flirty and always steamy!  Length: 60,000 words.  Spine colour: orange.
Blush: Are big romance novels filled with intense relationships, real life drama and the kinds of unexpected events that change women's lives forever!  Length: 85,000 words.
Featuring relatable characters who strike a chord with the reader regardless of the book's setting or plot points.  Length: 55,000-60,000 words.
Spine colour: purple.
Intrigue: Crime stories tailored to the series romance market packed with a variety of thrilling suspense and whodunit mystery.  Length: 55,000-60,000 words.  Spine colour: dark blue.
Desire: Contemporary, sensual, conflict-driven romances that feature strong-but-vulnerable alpha heroes and dynamic heroines who want love - and more! Reads that are always powerful, passionate and provocative.  Length: 50,000-55,000 words.  Spine colour: pink.
Romantic Suspense: These novels are romance-focused stories with a suspense element. Powerful romances are at the heart of each story, and the additional elements of excitement, adventure and suspense play out between complex characters.  Length: 70,000-75,000 words.  Spine colour: dark purple.
Nocturne: Dark, sexy, atmospheric paranormal romances that feature larger-than-life characters struggling with life-and-death issues.  Length: 80,000-85,000 words.  Spine colour: black.
As you can see, you can find all sorts of Romance novels.  Ultimately, they are all meant to transport a lady reader to a world of fantasy in which Good always prevails over Evil. Harlequin has similar guidelines.  But Harlequin also has a series of e-books, “Historical Undone”, with a length of between 10,000 and 15,000 words.  This could be a good entry point to test the waters before writing and submitting a full-length novel.
The series that I find more congenial is “Sweet Romance”.  This is because the novels are short (50,000 words) and don’t include explicit sex.  It’s not that I am so puritanical, but I don’t like to read about “sliding members” and “penetrating male sexes”.
Neither my wife and I have read Category Romance novels before discovering that they have such a huge market, but we are warming up to the idea of writing together Romance stories.
Here is our recipe for writing a successful “Sweet Romance” novel, taken from the writings of Valerie Parv and Emma Darcy, two very successful Romance authors.
Characters
If there is a character-centred genre, this is Romance.
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Romance novels essentially have  two characters: the heroine and the hero.  All other characters are  only there for support and shouldn’t do much. 
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The protagonist must the heroine.   She must be beautiful, intelligent, honest, and successful.  This  doesn’t mean that she must be perfect, but almost.  In essence,  she must be somebody with whom any woman might like to identify. 
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The hero must appeal to the vast  majority of Romance readers.  Therefore, he must be handsome, sexy,  A-male.  But he should also be not much younger and not much older  than the heroine and (obviously) honest, courageous, and generous.   In essence, every reader should be able to vicariously fall in love  with him.  Incidentally, surveys have proven that the readers prefer  dark-haired heroes. 
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The protagonists never  engage in casual sex, never steal, and never use violence.  It used  to be that the protagonist needed to be a virgin, but this is no  longer strictly necessary, although it is not appropriate to dwell  on previous sexual relationships of the protagonists. 
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If the hero does something  “naughty” like telling a lie or getting drunk, you have to  explain in detail why and show that he is in fact a good man and at  once feeling guilty for committing such a bad act. You should also  make clear that it is a one off and that it will never happen again.   Best, don’t make him do anything you then have to waste pages and  pages of contrition in order to recover from. 
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No swearwords, ever! 
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No physical features that would  make it impossible for the reader to identify with the heroine.   That is, the heroine must not be too tall or too short, with a  weight problem or anorexic, etc. 
Plot
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The plot should be linear.  Forget  flashbacks and memories.  They only distract from what is happening  now. 
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At least in the “standard”  50,000-word novel, no subplots.  There is not enough space for them  and, in any case, they distract from the main plot. 
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The protagonist and the hero  should already meet in the first chapter.  Possibly, in the very  first paragraph. 
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There must be at least one major  conflict between the protagonist and the hero, and this must become  clear as soon as possible.  Ideally when they meet.  This conflict  (supported perhaps by a couple of additional minor issues) is what  keeps the protagonists apart, even if they feel attracted to each  other immediately. 
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The main plot is the  evolution of the relationship between the protagonist and the hero  and the ultimate resolution of the conflict between them.  The  relationship should go through two or three crises, of increasing  seriousness, alternating with peaks of happiness/optimism, to reach  a satisfyingly happy ending. 
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The protagonists marry in the last  chapter, with a very short resolution, if any.  This implies that  you must resolve all minor issues and tie up all loose ends before  you resolve the main conflict and bring the protagonists fully  together.  Note that love, at least in Romance novels, is  forever. 
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Nowadays, they can have full  intercourse before marrying, but it shouldn’t happen too early in  the novel.  This is because intimacy is something to achieve, and  only when it is clear that the protagonists are in love.  Make them  do it too soon, and you will struggle to hold them apart until they  finally unite at the end. 
General Points
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As the readership consists almost  exclusively of women, you must not write what a woman is likely to  find distasteful, especially if it refers to the protagonist. 
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The point of view must be that of  the protagonist. You can briefly switch to the point of view of the  hero, but only if strictly necessary to support the plot and only  briefly and clearly. In other words, omniscient and multiple  viewpoints are out. 
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You have to maintain pace  throughout the novel.  This is done through dialogues and by  surprising or shocking the reader. Suspense and short chapters help.   Try to end each chapter with something that might encourage the  reader to start the next one.  These are short books, and many  readers go through one of them every day. 
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Try to set the novel in a  stimulating environment. Incidentally, novels set in the Australian  Outback seem to be quite successful with American readers. 
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Do not waste many words on the  scenery.  Ultimately, the readers are interested in the characters,  and in particular the heroine, more than on anything else. 
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Narrative should be kept to a  minimum.  Some readers page through books and buy those that contain  more dialogue. 
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Like with any other form of  writing, remorselessly cut down anything that doesn’t advance the  plot or help developing the characters.  With only 50,000 available,  you cannot afford long-winded descriptions or speeches. 
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Limit each chapter to about 20  pages, so that the whole book consists of 10 to 12 chapters. 
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Use short sentences in small  paragraphs.  A lot of ink without breaks is usually discouraging. 
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If you are a man, use a female pen  name and invent a persona to go with it, because almost no reader  will think that a male author can create a good female fantasy. 
To write successful novels, you always have to conform to what the readers want, and the readers of Category Romance have very strict requirements.  And this straightjacket is what makes it interesting for me.  I like the challenge.