i've been playing a few months and when i changed the strings i was reccomended to use extra light strings. is there a difference in sound with these and a set of medium? which ones should i keep on my acoustic?
yes there is a difference in tone, and sustain.
Thicker strings usually sound better, but are harder to bend and stuff. Thinner are the opposite. I don't play and acoustic, so I don't know what to recommend to you.
Make sure the truss rod is adjusted when the string tension changes.
Tonally, thicker strings will have more sustain and sound fuller. On acoustic you should go for the thickest strings you can while still being comfortable/being able to actually play the damn things.
It's generally accepted that thicker strings produce a louder more full tone than thin ones. While thicker strings may add a few decibels to the guitar I don't believe that they have any noticeable difference in tonal qualities in comparison to thinner strings. I usually go with light gauge strings because what I've found is the only real difference between heavy and light gauge strings is that light gauge strings are easier to play on.