It stands for the Account Planning Group and as far as I can tell is a bit like D&AD but for planners. Winning the Grand Prix is just like winning a Black Pencil. To win a Grand Prix, or a Gold or a Silver, you have to write a 2,000 word paper describing the rationale behind a piece of your strategy work.
Most people write these papers over several weeks, with 3 or 4 drafts.
Two planner friends of mine, Paul and Doug are entering the APG this year. They haven't written a single word yet. And the deadline is tomorrow - at noon.
This isn't down to their laziness or inefficiency. This is because they're writing about the something (can't tell you which client) where the strategy behind it was that people perform better under pressure. So they are going to write the paper in just 24 hours, live on a webcam. To place them under pressure.
You can help by increasing the pressure and watching them live over at Two Planners In A Room. The more pressure they're under the better the idea becomes. So spread the word.