Popular romance novels, as defined by the Romance Writers of America, contain 'two basic elements': 'a central love story and an emotionally satisfying and optimistic ending'. In other words, these are happy stories of (eventually) requited love, though there may be some suffering and many obstacles to be overcome before the lovers reach their happy ending. A slightly more detailed introduction to popular romance fiction can be found here, with a brief follow-up regarding the variety of sub-genres and countries of origin.
I have written two books about popular romance fiction: Pursuing Happiness: Reading American Romance as Political Fiction (2016) and For Love and Money: The Literary Art of the Harlequin Mills & Boon Romance (2011). I have also written a number of articles about popular romance fiction. I have my own blog on this site and, in addition, an index is available of many of the topics and romance novels I have analysed on the Teach Me Tonight academic romance blog.