An appositive is a noun or pronoun — often with modifiers — set beside another noun or pronoun to explain or identify.
The newspaper biz’s favorite use of the appositive is to put quotes around the name of a spouse … “Tom’s wife, Melissa, told him to turn off that damn computer and help out with the dishes.” See, you put the commas around Melissa because I’ve got just one wife, and leaving the commas off implies I’ve got others. (Pointless aside: I wonder how they deal with this at the Salt Lake Tribune … I’m guessing they’re a bit pickier about it in a city founded by polygamists. )