A person's mental energy is a consequence of both the motivational and emotional systems. Motives serve to develop an individual's psychic energy and to guide the person toward important tasks and goals. Emotions serve to amplify or diminish specific motives, and help integrate motivated behavior in a person's social interactions.
For example, the need to achieve will motivate a person to attain excellence in a given field. Emotions of pride and happiness may sustain such achievement; emotions of regret and guilt (for example, over harming others) can help ensure that the individual achieves while respecting the rights of others.
Motives -- This web site contains a classic 1937 paper by Gordon Allport on motives, which influenced much thinking in the field.
Human Motivation and Affective Neuroscience Lab -- People have conscious motives, but also motivational themes in their lives that they may be unaware of. One of the most interesting lines of recent research in this area concerns the degree to which people's conscious and non-conscious motivational themes agree or diverge. You can find out more at this site, led by Oliver C. Schultheiss.
Maslow's Hierarchy of Needs -- A brief overview of the theory.
Maslow's Hierarchy of Needs: Maslow's Original Article -- In Maslow's hiearchy of needs, each need can be viewed a single part of personality. The hiearchy itself suggests that needs can be arranged from those which must be satisfied most urgently to needs that emerge only later.