Romance novels, very popular in Western culture, are typified by a romantic relationship, generally between two or three people. Romance novels are centered on the development of the relationship between those contributing to it. Romance novels usually uphold a "happy" ending in which the good, generally the protagonist, is rewarded and the bad, generally the antogonists, are punished.
Suspense novels are written to perpetuate anxiety and bafflement about various outcomes of the plots within the text. Unlike romance, suspense novels can often focus on character development outside of the romantic realm; in fact, they generally focus on other components of a character's life. Suspense novels are known to be "page-turners," in that they perplex and interest readers as events unfold that begin to unravel these mystery plots.
Romantic suspense melds the genres of romance and suspense together, so that characters' romantic relationships are often the main focus of the writing; however, this relationship can elicit anxiety from the reader, or be a mystery throughout the text. Additionally, suspenseful subplots can be added throughout the text that elicit these same feelings. There is no tried and true way to discover whether a book can be considered a romantic suspense; a heart-lurching (due to both racy and suspenseful moments) page-turner is a good sign that it's a romantic suspense novel.
The following can be key themes within a romantic suspence novel:
The development of a romantic relationship between the protagonists
The development of a suspenseful/mysterious/suspicious situation between a protagonist and an antagonist
The advancement of some kind of intriguing element, such as a mystery, an adventure, etc.
The presence of a romantic villain as a central part of the plot
Olga Bicos, "The Heat of the Moment"
Lisa Gardner," The Third Victim"
Susan May Warren, "Nothing But Trouble"
Barbara Taylor Bradford, "Heirs of Ravenscar"