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.
Questions of personality have vexed mankind from the dawn of personhood: can people change? How do others perceive me? What is the difference between normal and pathological behavior? One's personality is so pervasive and all-important that it presents a clinical paradox of sorts: it is hard to assess our own personality, and impossible to overlook that of others.