To see all contacts that have a particular tag, start by navigating to the Contacts menu, then click on the Contact List tab. From here:

• Click the Filters button;
• Click the Select New Filter dropdown, and choose Tags.
• Keep the status as include, then click on the tag dropdown to see all available options.
• Choose the tag you would like to investigate, and click Apply Filters.

You will then be given a list of all contacts in your account that have this tag.

From here (after selecting any/all of them using the check boxes), you can also choose to address these contacts in bulk, by adding or removing tags, unsubscribing or deleting them, or downloading a list.