You can add tags when creating a new project—in the second step (Tags), or by editing an existing project in the Tags tab.

Selecting tags in a project
You can use tags in all types of reports—for example, in the Project List, Company Reports, Invoicing, Workload, Project Expenses, or Timesheet.

Filtering by Tags
Tag Filter
The tag filter also allows you to exclude projects from your search by checking the box Exclude results that contain any of these tags.