I keep hearing about rooting... is this like reinstalling Windows 7 on your PC?
lol...wouldn't necessarily phrase it
that way...
Have you read through this thread first?
http://androidforums.com/all-things-root-eris/45242-rooting-q.html
Rooting is basically giving you access / permission / ability to do and install things on your phone that VZW and HTC didn't intend on you necessarily doing. This would include things like
overclocking (making your phone run a little faster--albeit sometimes at the expense of using up your battery a little faster (although not necessarily so));
wi-fi tethering (i.e., making your phone into a wireless hotspot);
installing root-only apps that require special, admin-level privileges;
removing bloatware (apps you don't want/need); and other things.
There are several options for rooting nowadays (didn't used to be so). For example, you can simply add root capability to your phone as it is (you'll still have more to do to do some of the things I listed about like overclocking). Or, you can flash/install a whole, new custom ROM (replacing your phone's current operating system) which a developer has created that highlight certain features that the dev chose (like removing bloatware, special icons, built-in overclocking, different operating system versions, etc.).
Your best bet is to just start reading through the threads (in the
http://androidforums.com/all-things-root-eris/ subforum) to get an idea about what people are doing and saying about rooting. You'll learn a lot and start to form your own impression about what you might want to get out of it.
Hope this helps. Cheers!