PA = public address system. Alot of people plug their acoustic guitars directly into the PA (via a direct box) because, unlike electric guitar, a guitar amp is not a significant part of the sound of the instrument.
Acoustic guitar amps are specially tuned to amplify the sound of an acoustic guitar, while a PA is usually tuned to provide a flat frequency response in a given room. It is made to amplify vocals, bass, guitars, drums, and whatever else you plug into it.
A preamp, on the other hand, is an "amp before an amp." A mic preamp brings the very quiet signal of a microphone up to line level before it goes to the power amp which drives the speakers. A guitar preamp, in a tube amp for instance, lets you overdrive the preamp tubes and get lots of gain before going to the power amp section - so you can get lots of gain without a lot of volume.