A Tag is a label you can add to any of your campaigns. For instance, if you are running a radio ad on two different stations at the same time, you may choose different Keywords and measure the results against each other. But with more than two keywords it can get confusing.
This is why you can attach a Tag to each campaign ie. George FM and The Edge so they can be distinguished from one another in your Reporting:
You can choose to display your reporting according to Keywords or your Tags.
This is useful to measure one station's effectiveness against another. You can also use it to compare you ads in magazine, newspaper, TV or other mediums.