In the past I have sometimes refered to my gaming habits as Old School or Retro, but it occurs to me that they are not the same thing, but overlapping phenomena. Old School gaming involves turning back the clock to the non-existent Golden Age of Gaming. Old School is getting together with your high school game club and running your old PCs through module G3, because you never quite managed to finish the G series when you were kids. Old School is teaching your nephews to play D&D using the '81 Basic rules so that you can assure yourself that you helped raise them right. In short, Old School gaming is pure undiluted nostalgia.
Retro is different. The nostalgia shines through but it is tempered by modern sensibilities. A Savage Worlds game set in the World of Greyhawk is retro. Using the latest GMing techniques to drift your AD&D game into narrativism, that's retro. It would be the height of retro to run an old Traveller adventure exactly as written while ironically winking at the outdated sci-fi technology. Retro looks back at the Old Days, but remains apart.
At least that's how I define these two terms with regards to gaming.