This is how you send your game out into the public sphere. From the Crawl Steam Greenlight page, I can tell you that it's a "competitive multiplayer dungeon crawler where your friends control the monsters." From its detailed three-minute trailer, embedded below, I can tell you that it has beautifully animated pixel art, fast hack-and-slash combat, and lots of hot pink fonts. Crawl is about to get your attention.
When some games choose to reveal themselves with a single, terrible piece of artwork, or a bearded face asking you for money in response to some concept art, it's refreshing to see something like this. The Greenlight page goes into further detail: it's a single-screen action game, and the hero's job is to gather enough XP and loot to reach the game's boss. The boss, like every other enemy in the game, is cooperatively controlled by the other players of the game.
Maybe you can't watch videogame trailers where you work so here is a GIF:
I like these kinds of multiplayer games. Rather than straight up competition, where someone ends up on the bottom or has to sit out after death, here you and your friends can battle constantly but take it in turns to be the focal point of the action. From the video above, it looks like there'll be plenty of variation too, as advancing through the dungeon gives players new monsters to control, and those monsters evolve in much the same way as the hero levels up.
As for online multiplayer, developers Powerhoof are "exploring" the possibility but making no promises. They do say the game will have bots though, so you can play and practice even when you're friends are unavailable. There's more info and art on the Crawl developer blog.