This is probably the worlds most famous example of the tremolo style of guitar playing.

This particular piece was written by Francisco Tarega. It is performed here by Filomena Moretti.

If you close your eyes, and listen closely, it sounds as if there are two guitars being played. Watch the thumb and fingers on the right hand. The thumb plays one tune and the fingers another

Filomena Moretti. recuerdos de la alhambra. - YouTube

Here is another example of the tremolo style. It is performed by John Willliams. Filomena also has an excellent version on youtube. The title is Asturias and it is the fifth movement of the Suite Española. It was originally written for piano by Isaac Albéniz. It was later transcribed to guitar by Francisco Tarrega.

Asturias - Isaac Albeniz - YouTube

Both of these pieces are world famous and you'll find many versions by various artists on youtube.

Enjoy.