More than 300 amino acids are found in nature but only 20 amino acids are standard and present in protein because they are coded by genes. Other amino acids are modified aminoacids and called non-protein amino acids.
They are compounds of carbon, hydrogen, oxygen and nitrogen, where the four valences of carbon are attached respectively to an NH2 group, a carboxyl group, an H+ atom and a side chain, which may be polar or non-polar.
Free amino group is basic and a free carboxyl group is acidic. Hence, an amino acid is both an acid and a base and is called amphoteric.