Pastels are available in several types: hard, soft, oil and pastel pencils, each with their own unique characteristics. 

Pastels are made by mixing dry pigment, some chalk, and a binder together to form a thick paste. The paste is fashioned into sticks and allowed to dry. 
Because they are almost pure pigment, pastels produce intense colours that most other media can't match.

Artists' quality pastels contain the best pigments available and a higher ratio of pigment to binder. This means that artists' colours are stronger and more intense. They also have high permanence ratings, which means that they won't fade over time.

Students' quality pastels are much cheaper but usually contain cheaper pigments. They'll also contain more filler and binder, which means the colours won't be as intense, but they won't crumble as easily as artists' quality pastels.

https://www.art-is-fun.com/pastels/