Transaction tags provide another level of expense or income tracking.
While you may track your vacation in several categories, such as Lodging, Food, Gas, etc, a tag will permit you to easily track and run reports to determine the total cost of the trip.

To enter a tag, begin typing in the Tags field.

You can tag a transaction with multiple tags by separating them with a comma. For example, typing:

tag1, tag2

You can see a list of all your tags by going to the Tag Summary report.
This report is available within Tools --> Graphs and Reports, or by scrolling down to the reports in the left sidebar.

To bulk change any tags, you can go to Edit --> Advanced Search and search for a tag.
Then highlight multiple transactions and right click, select Batch Change --> Tags.
This will set the tags for all the highlighted transactions to the ones you enter in the Batch Change window.

You may wish to install the 'Find and Replace' extension within Extensions --> Manage extensions. Once installed, this extension will be available within the Extensions menu.
This extension will allow you search for transactions based on specific fields. You can then bulk rename tags by removing the original tag, and adding the newer tag.

Removing a tag from all transactions will delete this tag. Once deleted from all transactions, the tag will no longer be available for selection.