questions about rooting

No, rooting is not illegal, but it does theoretically void your warranty. However, if you need to return your phone and put it back to stock there is no way for Verizon to know it was previously rooted. This should answer some of your other questions. Visit the Hacks forum for much more info.

http://www.droidforums.net/forum/droid-hacks/632-what-root-means-stop-asking.html

 

Start with the Hacking/Modding section and read the guides, FAQ's etc. You really need to be able to seek answers on your own if you are interested in rooting, IMO. That said, there are plenty here that are willing to help if you have questions or get stuck.

 

They main things that you can do once rooted are overclocking your CPU and running different themes (color schemes, animations, other customizations). There are many other things that you can do too - read up like the others said to figure it all out. Also, use the forums' search function!