This screenshot should demonstrate the two different items. There is a tag, a "x number" and a set of followers. What do the different numbers mean?
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The number you see is the sum of the number of people following the tag via email, plus the number of people that have added the tag to their favorites.
A tag follower is someone who wants to receive daily email summaries of new questions in that tag. You can follow a tag by clicking the "subscribe" link on that pop-up.
The "x number" indicates how many questions have that particular tag. The most popular tags are on tens of thousands of questions.
For more details on this pop-up, see: http://blog.stackoverflow.com/2011/01/tag-favorites-and-tag-subscriptions/