How are essential and non-essential amino acids different?

The body is able to make non-essential amino acids from other amino acids found within the body. However, it isn't able to make essential amino acids. The only way to obtain them is by consuming/eating high quality protein foods. Protein sources that contain all of the essential amino acids are called complete proteins. Whey protein is a naturally complete protein. Incomplete proteins (such as vegetable sources) only contain some of the essential amino acids.