Tropes—when you're talking about movies or novels—are common themes, plot elements, or literary devices, so popular they've often become cliché. Some people hate them. Some editors will throw your manuscript in the trash the moment they see a trope they're tired of. Others will reject it because it doesn't follow a popular pattern. People like Terry Pratchett make an art of purposely employing too many tropes, to hilarious effect. The thing is, clichés become clichés for a reason: we, as a people, LOVE certain story elements, and don't mind if we see them over and over again. Entire genres are built on well established tropes that readers not only tolerate, but expect.
My opinion: be aware of the tropes of your genre, then go ahead and use the ones you like, the ones that serve your story, but play around with them. Make them your own. Mix them up. It's true that, when you boil everything down, there aren't a whole lot of truly unique stories. It's the way you tell it that makes it unique.
I'll be featuring individual devices and plot elements here in my Literature Tropes series, but if you want to get lost for an hour or two, visit tvtropes.org, where you'll discover tropes with creative names, like these, common in science fiction and fantasy:
Dark Lord on Life Support (A Bad Guy who's been wounded—often by the Hero—and needs a machine or a host body to survive. Think Darth Vader, Lord Voldemort, Stargate's Goa'uld.)
E.T Gave Us Wi-Fi (Where some or all of our technology actually came from aliens somehow. Useful for explaining why a human starship pilot, for example, can board an alien vessel and somehow figure out in two minutes how to control the ship.)
Clap Your Hands if You Believe (where the belief in something, such as magic, is what actually makes things happen. Think Tinkerbell, or go back even further, to the Bible, where Simon Peter can walk on water until he starts to doubt.)
Conveniently an Orphan. (Let's be honest: if someone up and answers the Call of Adventure, while leaving behind all family responsibility, he can come off as selfish, irresponsible, and unlikable. The solution: make him an orphan, so he has no family to leave behind. Bilbo Baggins and many other heroes of fairy tales and fantasy fit this bill.)
Token Heroic Orc (Where a member of a scary enemy species joins the—mostly human—good guys. Think Worf and Seven of Nine on Star Trek, Angel on Buffy.)