What are partitions used for?

I have problems with my disk drives on my computer and everyday it says i am low on disk space when i dont even do anything, and i delete all the programs i dont use and it still says i am low. So i am going to reinstall Windows XP, and i want to know what partitions are used for? because it says something about making a new partition or deleting one, and when i try to delete one it says your computer might not run right. How do i know which one to delete and which one to keep, because it has like 6 partitions? And when i make a partition how much space should i give it? Because last time i did it i got 4 hard disk drives. Please Help?