It sounds like you're looking for this page in the Splunk Documentation: http://docs.splunk.com/Documentation/Splunk/6.2.5/SearchReference/join
@ChrisG and the documentation folks do a very thorough job explaining the command (and the vast majority of other commands), and all of the options with it along with some alternatives.
You may also be interested in http://docs.splunk.com/Documentation/Splunk/6.2.5/SearchReference/SQLtoSplunk
But if you have a specific problem you're trying to solve with join, if you provide some description/samples of data you have, a description/sample of what you're trying to get to, along with searches that you've tried that didn't get you the desired result (and how they were off), someone would likely be able to give you more direction.
