Planetary Romance (also called Sword & Planet) is a subgenre of fantasy or science fiction in which an earthman will go to another planet, find the culture there savage and "romantic" in an action adventure kind of way, win the love of a beautiful alien princess, fight crazy monsters and dastardly melodramatic villains, and otherwise have a grand old time. For some reason, he's always the best swordsman around, and may have other advantages due to his earthling nature, too. The genre is mostly full of extremely formulaic and poorly written little mini-novels, but I'm a huge fan of planetary romance nonetheless. I'm not a huge fan of most of the actual books in it, but something about the genre itself speaks to me. I'm currently reading (in fact, I'm almost done, because it's a very short book. I blasted through about 100 pages last night and earlier today, and I've only got about 50 pages left) F. Gardner Fox's Warrior of Llarn which is a pretty standard example of one, coming out of the Second Wave of the genre. Here's how I divide the stuff up historically and thematically:
First Wave: Invention and Golden Age: The genre starts officially in 1911 with the serialization of "Under the Moons of Mars", later novelized as A Princess of Mars by Edgar Rice Burroughs. Burroughs himself was the most prolific writer of planetary romance, with a dozen Mars books, half a dozen Venus books, a series set on the Moon, and a final series that he died before he could get much work done with (Beyond the Farthest Star). Ralph Milne Farley and Otis Adelbert Kline are amongst the others who are part of this first wave, but even Robert E. Howard joined in (with Almuric) and it crosses over into other media with Flash Gordon. By the end of the first wave, it was merging with space opera, and heroes like Leigh Brackett's Eric John Starke straddle the line between the two similar genres. After the failure of the original pulp mags in the 40s, the genre quietly disappeared for several years.
Second Wave: Imitation and Reprint: In the early 60s, Burroughs, Kline, and others were rediscovered and put back into print, along with a lot of other pulp writers (sword & sorcery took off again big-time at this point as well.) A number of imitators popped up, including (but not limited to) Lin Carter (Callisto and Green Star series), L. Sprague de Camp (Planet Krishna series), Mike Resnick (Ganymede series), the aforementioned F. Gardner Fox (Llarn series), Michael Moorcock (The Kane of Old Mars series) and more. Leigh Brackett also trotted Eric John Starke back out (her older stories, revised and lengthened were put back into print) and wrote the best stories he appeared in; the Skaith trilogy. Other than her input, though, this wave was predominantly characterized more by open imitation and repetition of formula rather than by any innovations so much; many of the authors in fact openly acknowledged the fact that they were pure fantasies completely inconsistent with actual conditions on the planets on which they take place (Calliso and Ganymede, for instance, rather overtly do this.) Many of these writers failed to understand what made the original wave so fun in the first place (the wonder and imagination of it) and so along with formulaically repeating the flaws of writers like Burroughs, they also only weakly reflected his strengths. Few of these books are truly good, but they're interesting to read for a bit of historical perspective, I guess. This wave died out with the introduction of two very, very long-lived series that kept on meandering into print long after the subgenre itself had faded again; namely John Norman's Gor books (which later became more infamous for his bizarre and offensive take on sexism, but which started out innocently enough as simple Barsoom knock-offs) and Alan Burt Akers' Dray Prescott series (also known as the Scorpio series, or the Kregen series.)
I don't know if there's enough going on today to posit a third wave, but I'm impressed by books like S. M Stirling's Sky People and In The Court of the Crimson Kings which, while not in an of themselves great books, are at least modern novels with more sophisticated storytelling techniques, that openly and overtly are done in homage to the planetary romance of the past. I think the subgenre could stand this kind of revival; one in which good authors write with interesting characters, less melodramatic plots and a little more attention to verisimilitude.