Nylon Strings

From GuitarWiki

Most accoustic and electric guitars use steel strings, but classical guitarists usually use a different type of strings made out of nylon.

Nylon strings have a darker tone than steel strings do, and once you know the difference, it is easy to tell the difference just by listening to them. Nylon strings are also easier on your fingers because the material is softer and doesn't hurt as much for new players.

Nylon strings are usually only on spanish, or classical guitars. These two types are very similar, as Andres Segovia, the inventor of classical guitar used a Spanish guitar. This is ideal anyway, as the broader fretboard on Spanish guitars is ideal for playing classical music.

If you have a classical guitar it is very important that you use the exact same gauge of strings every single time you change them, or you can easily have a guitar that is permanently out of tune on some notes. It is also extremely important that you do not put steel strings on a classical or spanish guitar for the same reason.