Hello! I've seen acoustic guitars and electric guitar but I never saw any classic guitars. What do they look like?

Nakchura, the quickest ways you can tell the difference between a classical and a steel string guitar are the headstock and the strings. Classical guitars use nylon strings, and the top three strings (the three highest pitched ones) look like they're clear plastic rather than obviously steel. Also, the headstocks are different. Here's a picture to demonstrate.

A classical guitar has strings that are wound on pegs inside the headstock, like this:
http://www.barbarossa-guitars.com/guitar%20pics/ccl%20headstock.JPG

A steel string guitar, on the other hand, has metal tuning pegs that stand up on the headstock, like this:
http://www.candelas.com/images/product-c...r_headstock.jpg

You can also note the difference in how the strings look in the two pictures, and hopefully what I said first will make more sense.

Nakchura, I might add that classical guitars typically have smaller bodies than a standard "dreadnaught" size steel string acoustic, the the necks are wider and the fingerboard flatter and there usually is no pickguard on a classical guitar. Although a 3/4 size steel string is close in size to a classical as ShackMan said, the nylon or silk-wrapped strings are the best indication that you're looking at a classical. One word of warning -- I have seen used classical guitars for sale on which the owners have installed steel strings. This is a no-no, mainly because the tension of steel strings will play havoc with the classical's bridge and neck.

...and it happens all too often... =(