What are the differences between acoustic guitars and classic guitars?
There are several differences between acoustic guitars and classic (nylon) guitars. 
1) The strings in acoustic guitars are made of steel, but for a classic guitar they are made of nylon.
2) the second difference is in the sound and tone they are producing. The sound created by an acoustic guitar is more crisp and twangy as apposed to the one created by a classic guitar which is more mellow.
3) The third difference is in the shape and body. The neck of an classic guitar is wider.