People are just reffering to an "All Purpose Cleaner". This isn't a brand or particular product, but rather just a type of product.
You can find these anywhere from Canadian Tire, Home Depot, pretty much any general store like that.... Some folks will reccomend some more high end "APC's" but I just stick with the cheap stuff since I got through it like crazy.
For wheel well cleaning I use a 6 litre pestiside sprayer filled with a 3:1 cut of Generic Degreaser. Just put the wand in the wheel well, pull the trigger, mild scrubbing if required and then rinse + dress
Yep, Simple Green's an APC, and it works really well, you should just be careful about letting it sit for a while as it's supposed to be strong enough to eat through metal. A lot of people have pushed the move to a citrus-based APC for that reason...besides, they smell better.