An étude (said "ay-TOOD"; French for study) is a short piece of music written to help the player to become a better player. They are usually quite difficult. Sometimes they are just called "studies". Composers such as the pianist Carl Czerny and the violinist Otakar Ševčik wrote lots of études. Each one concentrates on a technical problem: fast scale passages, arpeggios, wide leaps, chords or, for string players, different ways of bowing.
The tradition of writing études started in the early 19th century, when the piano started to become popular. Many of the études from that time are still very popular today, including those by Muzio Clementi.
Frédéric Chopin wrote two sets of études which are very good for developing piano technique. Franz Liszt also wrote some very difficult études. These two composers' études are known for being not only good for training but also beautiful. Because of this, they are sometimes even performed in concerts.