MSG is the salt version of glutamic acid. Glutamatic acid is one of a chain of 20 amino acids that make up a protein molecule. It is a non-essential amino acid, which means that the body produces what is needed and we don't need to make it up in our diet. The brain uses glutamic acid as a neurotransmitter.
Glutamate is glutamic acid that has been broken down by fermentation, cooking or other methods. Monosodium glutamate is made by mixing glutamate with salt and water.