Proteins are polymers. The monomers of proteins are amino acids.
All proteins have the same basic structure. They consist of an Amino Group at one end, an Acid Group at the other end, and a Carbon in the middle which bonds with a Hydrogen atom and an 'R' group, which is specific to individual amino acids.
There are 20 naturally occurring 'R' groups, which corresponds to 20 different amino acids. Each different amino acid has a specific name. For example, Alanine's 'R' group consists of CH3.
Plants make all the amino acids they need themselves, as long as they can obtain Nitrate from the soil, which is then converted to amino groups and bonded to the products of photosynthesis.
Animals on the other hand cannot make amino acids themselves and so must take in proteins as part of their diet. These proteins are then broken down into amino acids that can form other proteins. However, some amino acids cannot be built from materials brought into the bodies of animals. These are called Essential Amino Acids, and must be eaten directly as part of the diet. Most of these can be found in meat.
Amino acids are toxic and as they cannot be stored, they must be excreted from the body in a process called deamination. In animals, this occurs in the liver, where amino acids are converted to urea and pass out in the urine.
Amino acids can be joined together, forming Peptide Bonds. All amino acids are joined in exactly the same way. A Condensation reaction forms a covalent bond between the monomers, between the amino group of one and the acid group of another. When two amino acids are joined together in this way, a dipeptide molecule is formed.