basic questions here by a really stupid guy

Okay some basic questions here by a really stupid guy. I found out yesterday that my car battery required water. I checked it and 2 of the places you put the water were totaly empty, the others had some but were not full. So I filled all slots and put the two covers back on. So my questions are:

1. Why do car batteries need water?
2. What happens if like in mine were low on water?
3. By having low water does this cause a strain on the engine/alternator at all? Quote:1. Why do car batteries need water?

Actually it is a sulfuric acid and water mixture in a lead acid battery. I am not a battery expert on the theory and operation of what exactly goes on inside a lead acid battery. You should go to a website such as HowStuffWorks to find out how they work.http