How does "The Binding Of Isaac" work?

There's 13 endings and about 80 secrets, along with 6 characters (I think) and like 200s of items.

It's not the real end until you unlock it all.

There are about 13 or so endings. I think it takes about 18 "full wins" to unlock all the content.
It's been a while but I believe floor level goes
Basement 1 -> Basement 2 -> Caves 1 -> Caves 2 -> The Deep 1 -> The Deep 2 -> Squishy Spoiler level 1 -> Squishy Spoiler level 1 -> Evil Spoiler Level OR Good Spoiler Level -> Spoiler Golden level.

The spoiler content is unlocked the more times you beat Mom. Each level has a "hard" alternative that can spawn. I think it's the Cellar for the basement. The Caverns for the Caves. The Necropolis for the Deep and there's one for the squishy spoiler level.
Sometimes levels can be spawned with a "curse" that can change the layout of the map.