The Relationships Report needs a little explaining because it has one significant difference from most reports: The same relationship may appear twice in the report. That's because the relationship belongs to two contacts.
For instance, in an Employer -- Employee relationship between Acme Industries and Wile E. Coyote, the report will contain a line in which Acme Industries is listed as the Employer of Wile E. Coyote, and farther down the report, another line lists Wile E. Coyote as an Employee of Acme Industries.
When the contacts are separated in the report the way Acme Industries and Wile E. Coyote are, this makes intuitive sense. If you're looking for Acme's relationships, look in the A's. If you're looking for Wile E.'s relationships, look in the C's.
Where it can be disconcerting is when the same relationship appears in consecutive lines. For example, if Shia Married and Heath Married are married, with a Wife -- Husband relationship, and neither has other relationships defined, you could have two consecutive lines in the report, the first listing Heath as the husband of Shia, and the next listing Shia as the wife of Heath.
At first this might seem like duplication of effort, but in fact it's still the same thing: they both have the relationship.
Sometimes your selection can be used to prevent showing both sides of the relationship. In the Acme -- Coyote example, you could do a personal search of your contacts to find all records where the name is blank but the company is not blank. Then run the Relationships Report and click the "Currently selected contacts" option. The report would list Acme Industries as the Employer of Wile E. Coyote, but would not list Wile. E. Coyote as the Employee of Acme Industries, because Wile E. Coyote is not in the selection.
With the understanding that the Relationships Report presents relationships from both points of view, it becomes a report much like any other. Besides filtering by selection, the Relationships Report also lets you choose which Relationship Types and Roles to include on the report, and lets you choose whether or not to include addresses and phones/emails.