Rooting directions vary from phone to phone, but a good place to start would be the Android Development forums at XDA-Developers (.com).
In and of itself, root only gives you access to secured parts of the phone, just like if you're using using Windows and you have to click that little Run As Administrator prompt. Basically it's a system developed a long while back that keeps the regular day to day operations from being able to have too negative an impact on the rest of the system.
The positive thing about root is everything it then allows you to do further on down the line including custom apps, configurations and even OS'.
Here's a nice little Wiki article that gives a little more detail:
Why Root - Android Wiki