Warning up front: TvTropes can make you lose hours – if not days – of your life.
To quote from the front page:
Tropes are devices and conventions that a writer can reasonably rely on as being present in the audience members’ minds and expectations. […] The wiki is called “TV Tropes” because TV is where we started. Over the course of a few years, our scope has crept out to include other media. Tropes transcend television. They reflect life. Since a lot of art, especially the popular arts, does its best to reflect life, tropes are likely to show up everywhere.
Tropes should be considered first and foremost to be tools. You could even call them the building blocks of fiction.
The site can tell you a lot about how stories work… or don’t. For instance, you could give a character the Idiot Ball, because they have to act stupid to move the plot along. However, if all the characters are and must be stupid for the plot to work, that’s an Idiot Plot.
As far as resources for writers go, TvTropes is unequaled in my opinion. It’s not just the thousands of tropes. They also have The Bad Writing Index, which can help you avoid pitfalls. There’s a page about creating a system of magic for your fictional world. There are pages of analysis that address things such as Why Are Guns So Rarely Used in Fantasy. There are hundreds of links about plot devices, and just as many about McGuffins, which are like plot devices, except they have no qualities in and of themselves that makes them important.
I’ve been wiki-walking through TvTropes for about a week. I’ve been calling it research. Sure that’s a little tongue in cheek, but really, it is true! Interestingly I haven’t found a trope about the kind of magic system I set up in my current WIP Unbreaking. That doesn’t mean my system is better. In fact, it may be worse, which is one of the things I’m researching. I also haven’t found an adventure trio like the one Unbreaking has. Again, this is something to look at and see what my alternatives are, and how the implications play out.
So, if you’re a writer, you need TvTropes. Just remember that it’s hard to climb out of. 😉