Brushes can be any set design that you can brush on to an image - be it grungy, splatter brushes, regular shapes, or whatever.
Gimp Brushes Photoshop Brushes Flow is the... way effects run with an image. For example, in that action-y image that Crai posted, of the ninja thing, you would probably have flow in which the effects are kind of... popping out at you. The ninja looks as if he's jumping towards you, so you'd have effects that emphasise that action - that flow with it.
Flow can also be used in a way to... lead a person's eyes. If you have a flow that goes from the upper left corner to the bottom right corner, a person will follow along with that line. So, use that with order of importance, sort of.
Static images, like the one you used in this sig, are harder to create flow with, because they're static. So, use action-y images - where the focal is doing some type of action, be it jumping, running, punching, or whatever. You can use flow to emphasise the focal's action.