Well there are 2 types of partitions. Primary and logical aka "secondary". There can only be maximum of 4 primary partitions and unlimited logical partitions on a particular hard drive. Usually it's a good idea to have one primary "/boot" partition and one root "/" partition and logical "/home" partition for most linux installations. And if you want to dual boot between windows have one primary partition for windows.
Reason to have a separate /home partition is if something goes wrong in your root partition you will at least have all your important documents, movies, music...in-tact. Also a separate boot partition ensures that your pre-installed windows boot record doesn't get messed up during installation. Linux will install boot record on a separate /boot partition instead of replacing windows boot record!!