Use joins to add extra data to GIS features in your map. For example, join income data to a parcel layer to add that information to those features. You can join data from the Internet, other organizations you work with, or GIS data repositories.
For example, after you join a table of voter turnout data to a map of regions in your area, you can create a theme that varies in appearance, depending on the number of participating voters in each region.
Typically, a join connects a separate table of data (a secondary table) to a feature class layer (the primary table). The most common type of join is a one-to-one join, which connects one record in a table of data to one feature in a feature class.
Typically, you join data tables from an application such as Microsoft Access to FDO layers from SDF, SHP, Oracle, and ArcSDE data. However, you can also join feature sources such as SHP to other feature sources.