In order for a novel to qualify as a romance, there are certain requirements it must meet:
The historical romance subgenre could be broken down even further into sub-subgenres: Regency, Victorian, and Gothic being the most popular.
Regency romances are set in England in the early 1800's during the British Regency period from 1811-1820. The first and original Regency romance author was of course Jane Austen (1775-1817), who actually lived during the British Regency. Some popular contemporary Regency romance authors are Lisa Kleypas, Tessa Dare, Eloisa James, Sophie Jordan, Sarah Maclean, Mary Balogh, and Madeline Hunter, among many others. This is the subgenre that is sometimes more colloquially known as "bodice-rippers".
Victorian romance novels are set in England (and sometimes the United States) during the reign of Queen Victoria from 1837 to 1901. Many of them are set during the later years of that period, which overlapped with the Belle Epoque in Europe and Gilded Age in the United States. Many of the Regency romance authors also write novels set in the Victorian period. Some authors particularly noted for their Victorian period novels are Joanna Shupe and Kerrigan Byrne.
Gothic romances are also usually set in England in the 1800's, but they are defined more by their plot elements than the time period. They usually take place in a dark, empty mansion and have elements of fear, foreboding, or the supernatural. One of the original authors of this subgenre was Ann Radcliffe (1764-1823). She was later followed by Charlotte Bronte (1816-1855) and her classic gothic romance novel Jane Eyre. Daphne du Maurier (1907-1989) and her classic Rebecca followed.
Contemporary romance is any romance novel set in the modern day. Some of the most famous and established authors in this subgenre are Danielle Steel, Nora Roberts, Debbie Macomber, and Robyn Carr. Some newer authors are Christina Lauren, Jasmine Guillory, and Alisha Rai, Sophie Kinsella, and Sally Thorne. A lot of contemporary romance novels would be considered the literary equivalent of a romantic comedy movie.
Paranormal romance is a very popular subgenre and is any romance that features supernatural elements. This is the group where all the vampire, shape-shifter, werewolf, fairy, demon, etc. romances go. Some popular authors in this subgenre are Charlaine Harris, Christine Feehan, Gena Showalter, Nalini Singh, Karen Marie Moning, Sherrilyn Kenyon, Kresley Cole, Patricia Briggs, Laurell K. Hamilton, and Deborah Harkness
Western romances usually take place in the modern day, but on ranches in the Western United States, and the main characters (always the male lead and sometimes the female lead, as well) are "cowboys" (or "cowgirls"). The plot complications usually revolve around property ownership and the difficulties of managing a ranch in the modern age. Some famous authors in this genre are Linda Lael Miller, Diana Palmer, and Maisey Yates.
Romantic suspense novels are contemporary romances that center around the investigation of some sort of crime or some sort of criminal threat. The threat and the investigation serve to bring the two main characters together and add an element of danger. Popular authors in this subgenre are Nora Roberts (sometimes under the pen name J.D. Robb), Sandra Brown, Lisa Jackson, Linda Howard, and Jayne Ann Krentz. You could also put Janet Evanovich in this category, except her novels are more comical and lighthearted, more adventure than suspense.