What is Protein?
Protein is material from which living bodies are formed and with which various chemical reactions are catalyzed. There are vast varieties of proteins and they may differ slightly, according to species (such as human and mouse) even when they play an identical role.
Physically proteins are polymer chains of amino acids. So-called genes almost always code the order of these amino acids (20 kinds of them are used). That is, genes are a plan for the proteins.
Structure and function of proteins
See also the general explanation "Structure and function of proteins" written by Haruki Nakamura, professor of Institute of Protein Research in Osaka University.