The Etude is also known as a Study. The Etude is a piece designed to teach a particular technique. It may also be that they were written solely to strengthen students' stamina.
Some techniques can include:
1. Running beautiful scales
2. Running in octaves
3. Running in thirds
4. Playing in staccato
5. Playing with a dominant left hand
Chopin wrote many Etudes for the piano. So did Czerny.