It is simply a tag you put on a deer after you shoot it to identify who shot it and when and what sex the deer is. It comes with your permit when you purchase it. Most people call the permits "tags" since they are on the same piece of paper usually. Now that I think of it, in MI the permit is the tag, you put the whole thing on the deer I think.

If you are driving around with a deer in your truck and there is no tag on it you are poaching pretty much.