Structural Classification Of Proteins
A database maintained by cambridge university of proteins organised into a hierarchy. The basic tree is :
Class, Fold, Superfamily, Family and Species
As with many protein classification sites, there are links to various other databases, such as SWISS-PROT, the PDB and so on.
Reference: Murzin A. G., Brenner S. E., Hubbard T., Chothia C. (1995). SCOP: a structural classification of proteins database for the investigation of sequences and structures. J. Mol. Biol. 247, 536-540.