Protein makes up the structure of most cells in the human body. The major function of protein with regards to exercise is its involvement in the reparation and development of all tissues and hence it is essential for muscle growth.
Protein is made up of small molecules called amino acids. While some amino acids are manufactured by the body, some must be obtained through the diet. These are known as the essential amino acids. All 22 a.a must be present in order for the body to metabolize protein properly.

Complete protein: contains all 22 essential amino acids (also termed high-quality protein). It is common for animal sources to be complete proteins.

Incomplete protein: contains only some of the 22 essential amino acids. It is common for plant sources to be incomplete proteins and hence fail to support growth when used as the sole protein source.
How much protein do I need?
To determine your personal recommended daily intake of protein, simply multiply your body weight in kg by 0.8g. (Note however that larger individuals, pregnant women may need slightly more protein.)
Eg: Joe weighs 165lbs
(Recall: Convert lbs to kg by dividing by 2.2!)
=(165/2.2) x 0.8
Therefore Joe requires 60 g of protein per day

In other words, remember that approximately 20% of your diet should be comprised of protein. Please keep in mind that if you are resistance training you can increase your protein intake.

What about protein supplements? Click the protein link at the top of this page.