Motivation is an important part of psychology. It is the driving force by which people (or animals) get what they want. A motive is the reason for an action taken.
Motivation arouses a person to act towards a desired goal. It is a driving force which promotes action. For example, hunger is a motivation which causes a desire to eat. "Motivation is an energizer of behavior".[1] It is the purpose or psychological cause of an action.[2]
With animals, motivation is caused by basic needs: needs for food, water, warmth, safety, mating, protecting the young, defending territory, needs to escape pain and threats... The drive to do these things is instinctive, inborn, and triggered by circumstance.
With humans, whose mental life is much more complex, motivation is more complicated. Obviously, humans feel the need for food and water, avoid pain and so on. But they are also capable of having long-term plans which are more difficult to understand.