SCOP (Structural Classification of Proteins) is a classification of protein structures in PDB. Since it is a classification, the annotations on structure and sequences databases use references to SCOP to describe their proteins.

Depending on exactly what you are looking for you can use ASTRAL to retrieve sequences corresponding to SCOP domains or you can can query PDB to find structures matching SCOP terms (see the "Advanced Search" and select "SCOP Classification Browser").