Rooting
Rooting is the process of gaining administrative rights to the phone so one can do customizations that aren't normally allowed. Root by itself doesn't do anything interesting, unless there's an app that needs it. The fancy stuff comes from custom ROMs. Just remember Virgin Mobile Activation only works on Froyo ROMs, so do it before going to Gingerbread. The easiest way to root nowadays is to use GingerBreak. It can be found here:
[26.04.2011][v1.2] GingerBreak APK (root for GingerBread) - xda-developers