Quite simply, cognition refers to thinking. There are the obvious applications of conscious reasoning—doing taxes, playing chess, deconstructing Macbeth—but thought takes many subtler forms, such as interpreting sensory input, guiding physical actions, and empathizing with others. The old metaphor for human cognition was the computer—a logical information-processing machine.
Self-control separates us from our ancient ancestors and the rest of the animal kingdom, thanks to our large prefrontal cortices. Rather than responding to immediate impulses, we can plan, we can evaluate alternative actions, and we can refrain from doing things we'll regret. We can also take advantage of these innately human abilities by developing wisdom and willpower.