You can join two or more related data sources to create a larger integrated data structure from which you can report in a single request. The joined structure is virtual. It is a way of accessing multiple data sources as if they were a single data source. Up to 1023 joins can be in effect at one time, for a total of 1024 segments, depending on the number of active segments and the number and length of the fields (there is a 32K limit on the length of all fields).
For details about data sources you can use in a join, see Data Sources You Can and Cannot Join.