What is casein?
Casein is a type of protein.
Milk is composed of two different types of proteins, one of which is whey protein, also known as whey, the other is casein, at a ratio of 20:80, i.e. 20% of milk protein is whey and 80% is casein.
Like whey protein, casein is also a protein of high biological value, although inferior to whey, and as such contain all essential amino acids.
Casein, unlike whey, is a slow-absorbing protein and is therefore commonly used before bedtime, as it will provide amino acids to the body for several hours, which is ideal while we are sleeping.
It can also be used in other meals to increase protein content, as in the case of whey.
You can find good examples of casein supplements here.