Rooting, in and of itself doesn't really do anything to the phone. All it means is that you have 'super user' (SU) privileges. It gives you the ability to completely customize your device from the ground up. This can mean anything from changing the color of the capacitive buttons (home menu search and back) to overclocking custom kernels (the foundation of your phone's operating system or ROM) to increase performance. The other advantage of rooting is the ability to use certain apps that require it. The first question that you need to ask is WHY you want to root. If it's just to customize what you see when you hit the home button, or the lock screen, you don't really need to. Go Launcher, ADW launcher, and Launcher pro are all free and will completely change the look of your home screen and app drawer. If you want to do more, such as use a custom ROM or increase performance, then it may be something to consider. The first thing I would advise is to do a LOT of research.
Addictive Tips - Tips to make you smarter! Android & Windows Phone: Tablets, Apps, & ROMs @ xda-developers are two great sites. Start with Addictive tips as they put it more into laymans terms. One other thing to keep in mind. Rooting voids your warranty, for a reason. And, while it's not likely if you follow the instructions of someone who knows what they are doing when you root and put a custom ROM on your phone, bricking is a possibility. If you aren't sure ask BEFORE you do ANYTHING. Hope this helps.