Just a warning: this game is messed up, in a way I can't really describe.
This game is essentially a cross of a roguelike and the dungeons from the original Legend of Zelda. Your initial weapon is your character's tears, which can be upgraded numerous ways via in game items. The way your weapon can be upgraded can be a little weird as well, there's your standard range and damage items, some normal, some strange, like mom's lipstick. And then there's the ones that change your weapon into blood, spectral tears or other various types of tears each with their own advantages. Spectral, for example can go over rocks and obstacles to hit enemies.
Each level is randomly generated and your character goes through rooms and fights enemies. Every level has a boss room and a treasure room, there are keys and bombs, secret rooms can be found using bombs on walls, in fact most of the basic stuff of the levels is the same as Legend of Zelda dungeons. Some levels have a shop where you can buy items (which the game chooses at random) or an arcade with a few choices of games you can play to earn coins, health and other goodies.
Bosses are also random, it seems that the game has a list for each part as I have not seen the same bosses in the basement levels as in the caves levels (and the caves always have harder bosses).
There is also a lot of content in the game, they boast over 100 unique items, some of which change the appearance of your character. Initially you start with just one character (Isaac) but you can unlock 4 others. They say there are over 20 bosses, 50+ enemy types, each with the ability to be "special" making them harder but also making them drop better loot. Apparently there are multiple endings and 4 full chapters spanning 8 levels.
I personally enjoy this game when I've got time to kill, I usually make it to the third or fourth level, but I always have fun, and am usually weirded out by something.