I've read a lot about preamp tubes and power tubes but what are output tubes? Do they need to be replaced? After how long? How do I know when they need replacement?

"Output tubes" is just another way of saying "power tubes".

...not to be confused with the transformers where "power" and "output" are referring to separate components.
ESP LTD EC-256 and a Fender Deluxe VM
Thanks it helped me a lot. Do you know why Fender sell pair of output tubes like 6L6 Groove Tube but they don't sell 12AX7 AND 12AT7 preamp tubes in matched pair?
preamp tubes do not need to be matched. In fact, you can use 6 or 7 different preamp tubes depending on what tones you are looking for.....
I've bought, sold, and traded more gear than I care to admit.
Easy way to look at it......A tube amp will have two sets of tubes, one will be the preamp tubes *this is where the amp goes thru the equalisation, gain stage etc etc basically where the meat and potatoes of your tone comes from. Then the second set power tubes these are the guys that send the signal from the preamps into the speakers so it can give you an eargasim.
"People worry too much about tone instead of just rocking out"