In BridgeDb, a {@link org.bridgedb.DataSource} defines an online resource such as Ensembl or Entrez.
An {@link org.bridgedb.Xref} is the combination of an identifier and a particular resource, and thus uniquely defines a single biological entity. It is often possible to generate a URL to an online description of this biological entity.
Identifiers are preferably not descriptive. A descriptive name such as INSR (Insulin Receptor) instead of Entrez Gene 3643 can be useful. In BridgeDb we refer to this as a Symbol. Because of the ambiguous nature, Symbols are not the primary means for identifying
Usually, different DataSources describe the same biological entities. Cross references are stored in so called Synonym databases. {@link org.bridgedb.IDMapper} has facilities for mapping cross-references and mapping between different DataSources.