A wash is a mixture of paint and water (or whatever thinner you chose). make it very thin and put it on the model, it will run into panel lines and crevices, adds a depth to the model. Once its dry you can wipe it off and a small amount will remain. Makes the model look grimy most of the time I do it.
Drybrushing is a method where you dip your paint brush in some paint, then wipe most of it off, and go over details. A tiny amount of pain will come off on the raised panel lines, I use this when I do cockpits as it helps make the dials really stand out.