Doors and Windows - More ?'s on replacing interior doors

I have some ugly hollow interior doors that I'd like to replace. How do I go about doing this? Can I just measure the existing doors and go to a building supply store and find new doors that match the measurements and then just hang them? Or do I have to replace the entire frame as well?
Also, I have heard the term Pre-hung doors but I'm not quite sure what that means? Can you tell I'm clueless about this stuff? :)
Susan, with the ugly doors

You would probably be better off just stripping and refinishing the existing doors, but here is my answer to your questions, anyway. Your call.
"Pre-hung" doors come with the complete frame with them.
Believe me, if you're not experienced at building door frames and hanging doors, you are MUCH better off going with them, and they cost about the same as just buying a door by itself.
To remove the old, remove the trim carefully, so that you can try to re-use it, remove the door from its hinges, and sink the nails with a nail set to remove the frame. Keep the trim, hardware, and doors (they make great computer tables, etc.), and toss the old frames.
You will need shims (2" splits of beveled cedar shingles work well), a level, hammer and #8 finishing nails to hang the new door frame.
Start by raising the door and frame up high enough into the rough opening to clear carpeting/flooring by putting small blocks under the frame ends.
Next, get the butt (hinge side) of the door plumb by shimming behind the hinges from both sides. Nail through the shims, and cut/break off whatever shim remains sticking out.
Next, level the top with shims and nail. Then the other side. Check that the door hangs plumb and level, and remains in place just where you leave it, and that it closes with about the same clearance all around.
I'm not a carpenter, but that's how I do it. If you have any problems, just come back in here and ask one of the pros.
Good Luck!

What Old Guy told you is accurate. You could take you old doors to a door shop and have new ones made to match (hinges mortised, knob holes bored, etc.). That will cost you as much, if not more, than simply buying 'prehung' doors and installing them yourself. Refinishing the ones you already have is by far the least expensive and least labor intensive.

Hi Old Guy and Lefty,
Just wanted to say thanks very much for the great advice and info. One thing I have decided is that I wont be doing this by myself! I'll either refinish the exisiting doors or hire a professional (I'm just not that talented) :) thanks again!
Susan