It's a community where fans of a specific fandom* come together to participate in writing, graphics, media, puzzle, luck and trivia challenges to accumulate points for their teams. The team with the most points, of course, wins the current round and may receive a celebratory banner as well as bragging rights. It's also a lovely way to make new friends in the LiveJournal world. If all that still sounds a bit confusing, don't panic. Most land comms usually provide a Rules & FAQ page (like the one I'm citing from cw_land).
* I say this loosely, as I've encountered land comms for specific fandom genres, too. Like spy_land and scifiverse. They normally provide a list of fandoms you can use for the challenges.
What kind of land comms are out there?
Good question. I've been a part of a little more than ten so far, and that still isn't all of them. I guess it depends on the fandom, how popular it is and how active the fanbase is. Here's a list of the ones I've joined, as well as some other miscellaneous things I'd like to keep track of: