According to wikipedia....
* Primary structure - the amino acid sequence of the peptide chains.
* Secondary structure - highly regular sub-structures (alpha helix and strands of beta sheet) which are locally defined, meaning that there can be many different secondary motifs present in one single protein molecule.
* Tertiary structure - Three-dimensional structure of a single protein molecule; a spatial arrangement of the secondary structures.
* Quaternary structure - complex of several protein molecules or polypeptide chains, usually called protein subunits in this context, which function as part of the larger assembly or protein complex.