Guest: It's perfectly simple: an "acoustic" guitar is a guitar of the traditional type...hollow body, so that the sound is produced acoustically, whether or not that sound is then further amplified by microphones or electronic pickups.

An electric guitar is a guitar with a solid body, and the sound is not produced acoustically to any very noticeable extent, but ONLY through electronic pickups.

Then, to split hairs, you have some acoustic electric type guitars which are sort of a compromise between the 2 above: they have a shallower hollow body or partially hollow body with some acoustic sound, and they also have pickups, but they are almost always played using the electronic amplification provided by the pickups.

If a "rock band" goes "acoustic", then it means that they are playing on acoustic guitars, not electric guitars. Whether or not there are pickups in those acoustic guitars is not the point...the body type of the guitar is the point.

Acoustic guitars sound quite different from electric guitars, they look quite different and they feel quite different to play...you need more force to press down the strings on an acoustic, so the technique is quite different from playing on an electric.   Acoustic guitars are larger in the body, much lighter in weight, and warmer in tone than an electric guitar. Either type can of course be amplified to any level of sound you want, if you use electrical accessories to do it.

Many of Dylan's purist folk fans in '65-66 thought a folksinger shouldn't be playing an electric guitar at all...they considered it to be a commercial "sellout" at the time...the act of a "traitor" to the folk ethic! ;-) Times have changed. There were hardly any of them who knew he had greatly enjoyed fronting electric rock bands in high school, and was simply returning to something he'd done before. Why? Because he felt like it at the time, that's why. He enjoyed doing the electric music, and he had gotten bored with the solo folk thing after about 4 years of doing it intensely. He must have been "sick of all this repetition", to quote a line from one of his songs.