There are basically two types of proteins that are of concern to us:

Complete proteins: Protein that contains all of the essential amino acids in adequate amounts.
Incomplete proteins: Protein that lack some essential amino acids.

Most foods of animal origin (meat, poultry, fish, eggs, cheese, milk) contain complete proteins. Plant sources (fruits, vegetables, grains, and nuts) do not always contain complete proteins. In fact only one plant protein, soy protein, contains all of the essential amino acids. However, it is possible to combine several incomplete proteins to make a complete set of essential amino acids.

Protein quality is measured in Biological Value (BV). It summarizes how readily the broken down protein can be used in protein synthesis in the cells of the organism. BV does not take into account how readily the protein can be digested and absorbed.

Food	BV
Eggs (whole)	100
Eggs (whites)	88
Chicken / Turkey	79
Fish	70
Lean Beef	69
Cow's Milk	60
Unpolished Rice	59
Brown Rice	57
White Rice	56
Peanuts	55
Peas	55
Soy beans	47
Whole-grain Wheat	44
Corn	36
Dry Beans	34
White Potato	34

The biological value is often shown as compared to egg protein. As a result, some protein supplements have a higher biological value than 100.

Food	BV
Whey Protein Isolate	159
Whey Protein Concentrate	104
Casein	77
Soy Protein	74

The higher biological value proteins will be better absorbed and provide more amino acids which can be used for the body's various needs. With that being said, it is important to consume a variety of protein sources in your diet.

Some lower biological value proteins will be digested slower and can help increase satiety further. This can be a benefit depending on when it is being consumed.