Wikipedia: Classical guitar

Musical instrument from the guitar family. Also called Spanish guitar.

The classical guitar is distinguished by a number of features.

It's an acoustic? instrument. The sound is amplified by a [sound box]?.
It has six strings.
The strings are made from cat gut?, or much more commonly these days nylon, as opposed to the metal strings found in some other forms of guitar. These strings have a much lower tension than steel strings.
Because of the low tension of the strings the neck can be made entirely of wood, not requiring a steel truss road.
The neck tends to be broader then with steel string guitars, making more complex work easier, but requiring a left hand position which ultimately makes the guitar less stable to hold.
The strings are usually plucked with the fingers.

The heyday of the classical guitar repertoire lies in the 19th century. Some guitar composers are:

Because of the relative scarcity of pieces, guitarists often play transcriptions of music originally written for other instruments.

Classical guitar players: