Several protein sequence and structure databases have emerged from a worldwide effort to curate the information on protein sequences and their structures. Sequences are written in different formats and certain standards have evolved. On the other hand, Protein Data Bank is the largest source of information on protein structures, and sets standards on the way structural information should be written. For handling large number of protein structures and sequences in bioinformatics, a thorough understanding of the formats and contents of these databases is required. In this review, we deal with some of the most common sequence and structure databases. Several protein structure databases have emerged, dealing with special issues and varieties of protein structures. We survey the contents and availability of different databases and review their classification and mutual linking. Certain quality and financial issues on these databases are also touched upon.