What is the difference between a Acoustic Guitar & a Folk Guitar? Also, if you can please. Is it "an acoustic" or "a acoustic" ? Thanks.
Thanks. I meant why are they called "Folk"? If I go browse guitars i will find a category for "Acoustic" & "Folk". Why? Thanks again.

An acoustic guitar is any guitar that has a hollow chamber that makes sound entirely acoustically without need of amplification. There are lots of kinds, including steel string flattops, archtops, and classical (nylon string) guitars.

"Folk guitar" is essentially a meaningless term that is typically used to indicate an acoustic guitar with a smaller body than the more common dreadnought or jumbo size. Something like a grand concert or even parlor size acoustic guitar might be called a folk guitar. The term presumably comes from the fact that many folk musicians – myself included – prefer smaller body guitars.