Primary structure is the sequence of amino acids found in a protein molecule.
Secondary structure is the coiling or folding parts of a protein molecule due to the formation of hydrogen bonds formed as the protein is synthesised. The main forms of secondary structure are the α-helix and β-pleated sheets.
Tertiary structure is the overall 3D shape of a protein molecule. It is the result of interactions between parts of the protein molecule such as hydrogen bonding, formation of disulfide bridges, ionic bonds and hydrophobic interactions.