Rooting can be very risky if you do not know what your'e doing. It is like jailbreaking an ipod/iphone. You hack it so that you can gain full access to the entire phone. You can customize it, alter cpu speeds, and delete apps, files, and data that you otherwise couldn't if your phone is just running the stock operating system.
It does void your warranty, and if you do something to break it, you will not be able to get a replacement or a fix from verizon, unless you are able to get into it an unroot it.