An appositive is an interjection into a sentence. While this part of a sentence may provide useful information, the thought expressed by the sentence will stand fully on its own without the appositive.

For example, in the following sentence "my best friend's collie" is an appositive:

The dog, my best friend's collie, caught the frisbee every time.

While the appositive gives the sentence additional information and character, it is not necessary to comminicate the primary thought: namely, the dog caught the frisbee every time.

Appositives are always separated from the main body of the sentence by punctuation, usually commas, but sometimes--when greater seperation is desired--dashes are used.