In addition to setting a person's access rights, you can further limit the data they see by campus. For example, a person may have access to see all people in the system. However, if you add a limit for one or more campuses, they will only be able to see people who are part of that campus.
To add a limit on campus, go to the person's profile and manage their permissions. In the popup that appears, click on the "Campuses" tab.
If "Limit access?" is set to No, the user will have access to all campuses in the system. If set to Yes, they will be restricted to only campuses that are checked.
Things to keep in mind:
In cases were campus is optional on a record, that record will be accessible regardless of these settings.
A person will always have access to their primary campus. (i.e. the value in the "Campus Attended" field on their profile) regardless of these settings.