Heh, it actually depends. There are "sheer" curtains, which are mostly see-through, and for decoration.

Then there are average curtains, which block some of the light. (I have those in my livingroom.

Then there are heavy curtains, which are specifically made to block out light and absorb noise. I have them in my bedroom. Even during the day, the room can be made pitch black and pretty much into a Man Cave. :-)