What actually distinguishes a classical guitar from any other guitar is the shape of the guitar. On a classical guitar the fret board is wider than steel-string guitars. Also, the body of the guitar (assuming the guitar is not a cutaway) comes to the twelve fret of the guitar. The body is quite a bit smaller than the usual big steel-string guitars. The guitar can be strung with nylon stings or steel strings. Most commonly it is indeed strung with nylon, but there are some guitarist who prefer the tone of the steel strings. Prior to either nylon or steel they made the strings out of the guts of animals, usually lamb. The classical guitars nick name is actually the gut string guitar, which is just an oh so pleasent thought.

The shape of the guitar is the only thing that makes it different. The style of playing classical guitar would be an entirely different thing.

Note: I do not suggest stringing a normal classical guitar with steel strings. Classical guitars are usually not built for that kind of pressure. Although, some guitar companies make classical guitars with the ability to withstand the pressure of steel strings.