Romance, as a genre, has a wide variety of categories: mystery, paranormal, contemporary, historical, erotic, courtship, blah blah blah. The book's focus is on the hero and heroine. They tend to be very dramatic and melodramatic. Sometimes there's an outer villain preventing the couple from getting together. Sometimes it's just society.
I've read far too many of these books, so I'm just going with a favorite few.
Sherry Thomas' Not Quite a Husband features a lady doctor and her mathematician-wiz husband, who were married, annulled their marriage because of the Deep Dark Secret, and now find themselves in a remote spot in India, struggling with each other and their surroundings. It takes place in 1897, there's a rebellion against the colonially repressive English, and a battle. But because it's romance, the romance, both past and present, is at the forefront. Everything in the plot serves to highlight the characters and how they should be together. I felt that this one was different from others I'd read because the woman was the relationship- and commitment-phobic one and the guy was surprisingly...not a manwhore.
To Rescue a Rogue by Jo Beverley was the last book in a very long series. So, there were past couples from the other books showing up, there was an established and specific time setting and the plot was carried over from the what male lead had experienced in past books in the series. Now he was getting his own story. This one was pretty dark for a romance novel, only because the hero was dealing some serious issues. But I found the end and the love story satisfying.
Lord of Scoundrels by Loretta Chase has been rated at the top of many a "Best Romance" list. It takes place in the 1820s. The hero is a half-Italian, half-English marquess with a dramatic personality and quite the reputation. The heroine is a no-nonsense young woman, almost "on the shelf." She doesn't try to change him, only to understand him, and the situations they end up in are more the realistic end of the spectrum.