Wake: Those waves that come out from behind boats. (Water and air are both fluids, and behave similarly - air is just a lot less dense.)
It really is the same as boats on water - a large slow moving boat leaves a very small wake. A small fast moving boat (like a jet ski) can make a decent wake despite being small. It's the same with cars. A semi passing slowly won't suck/push nearly as much as a Prius passing at high speed.
The large rental box trucks seem to have the worst wake - probably from those large cab-overs, and the people driving them tend to go fast.