Spelunky is a permadeath platformer, it feels very much like the old mario games, but the gameplay is quite different.
It's all about collecting gems and equipment, which boost your abilities, in a randomly generated level with some random events mixed in.
A game about exploring! Which means, be careful reading in forums and don't look at guides, only if you're really stuck, otherwise you will spoil your experience!
For those of you, who don't know the original: Spelunky was a freeware game originally, so if you're on the fence on buying the game, just download the freeware version first to have a look.
Note also that Terraria was inspired by (the older) Spelunky. They have built in many references to remind people of their roots (e.g. "a horrible chill goes down your spine")
The game is really tons of fun and very addictive! I can't wait to try out the local(!) coop mode with my friends :)
Btw, thx gog for being such a fine source for drm-free games!