Joined reports are one of the most powerful and underutilized tools Salesforce has to offer. With a joined report, you can show data from multiple objects on one page grouped by a shared relationship. Creating joined reports is a little tricky at first, but in principle it’s just like creating any other report. Drag and drop fields, summarize data, filter based on various criteria. You’ll have a section of common fields which allow you to group the data from the various objects together. Want to show Opportunities and Cases related to an Account on one page? Joined reports can give you this kind of 360 look into an Account in no time flat!
Now for the limitations. Joined reports don’t let you break down your data the same way as standard reports, nor do they let you filter data the same way. There are also some minor UI differences. You can’t add bucketed fields. Cross filters are a no go. Conditional highlighting isn’t an option. Also, you can’t drag and drop fields directly into the filters section like you can with standard reports (a minor inconvenience). For a full breakdown on joined reports including all the dos and don’ts, check out Salesforce’s article How Joined Reports Work.
-Jared and the Salesforce Guys