Category romance is also known as "Series" romance, depending on who's doing the talking. The term "category romance" derives from the fact that the books are published in clearly delineated categories, with a certain number of books being published in each category every month. Their alternative name, series romances, came from the sequential numbers sometimes printed on the books' spines. Category romances are short (usually no more than 250 pages), and have a low purchase price compared to other fiction books. These series or category books are also often considered to be one of the many romance genres.
Category became the hallmark of Harlequin and Mills & Boon who created romance "series" that were published every month. The publishers would release four or more titles under a common "line" name, such as Silhouette Intimate Moments every month. Although called a "series" these books are not necessarily related by a common topic and are not related. They are issued with numbers identifying the order of their publication.
Each line has its own personality and tone, ranging from Sweet to sexy. Some lines focus, for example on suspense, Harlequin Intrigue is that publisher's romantic suspense line, others on Medical Romance, and others on family issues. Others lines are distinguished by their word count.
Regular readers of category romance pick up these books with clear expectations for the kind of tone and story they will be reading. Other hallmarks of category include shorter length (though Harlequin Superromance comes close to Single Title length), standard look-and-feel in packaging, and a set number of titles published every month.
In today's market, category romance is synonymous with titles published by Harlequin. However, category romance is often used to describe a shorter story where the building romance between the hero and heroine dominates the story.