Hi! When you have the custom recovery you're done with all the hard stuff.
1. You don't have to bother with any of those yet, and if you want to know most should be quite easy to search for and fins an explanation.
2. You don't have root access until you either enable it using the recovery menu, or until you flash a custom ROM with built in root.
3. Galaxo 1.6.3.4 is definitely the most stable Galaxy ROM today.
Nandroid backup is a wonderful thing as it allows you to get back to the exact state you were in before starting flashing ROMs or themes. Just remember that any messages or the like you have received since you took the last backup will be gone if you revert.
Hope this help.