FogBugz tags allow you to assign searchable keywords to cases and wiki pages. To assign tags to a case, just start typing a tag name into the "Tags" field in the case view. As you type, you'll be presented with "auto-complete" results that allow you select an existing tag:
You can select one of these tags, or create a brand new one by typing it in completely and hitting the space bar or tab key (tags cannot contain whitespace).
You can also add tags to a wiki page by editing the page and entering tags into the "Tags" field at the bottom of the page:
Use the "Select Columns" control in the grid view to view the tags column. Now you can easily see how cases are tagged, as well as sort by tag:
Searching by tag is especially useful, as it provides a means of "instant recall." For example, if you have a few cases you'd like to discuss in a weekly team meeting, you could tag them all "team_meeting." Then, at the meeting, just search for "tag: team_meeting."
Using the "Tags" screen (accessed by clicking the Tags item in the Reports menu), you can view an easily searchable list of all tags in use on your FogBugz site.