From what I understand downloadable content is extra content that you are able to purchase and then download where as add on content, is already stored on the disc and is unlockable through payment. EA is pretty famous for this.
same thing, different name.....
Or you can look at it this way....
Add on content is adding on to the game(more maps being added to BO2, as an example....)
Downloadable content is whole new levels being added(like having a new world in New super mario bros, etc.)
You mispelled "acitivison is pretty famous for making their costumers pay for stuff that's already on the disk and EA is famous for trying to take over the world with the EA network thing that even infected the Wii. Report system, gamertags. It was all on the wii. Thanks to EA. And exclusive to EA games...."