The limbic system is a grouping of structures in the brain that sits between in the most primitive part of the forebrain called the rhinecephalon. This is a doughnut-shaped network of neurons that inlfuences many deep-rooted drives and emotions including pain, anger, hunger, sex, thirst, and pleasure. The thalamus, hypothalamus, amygdala, fornix, mammiliary bodies, and septal areas are all parts of the limbic system.
James Olds and Peter Milner discovered the function of the limbic system accidentally in 1954 when experimenting with electrical stimulation of the brain using rats and lever pressing. They found that when they stimulated this area of the brain while the rats pressed the lever, it produced pleasure and in response, the rats kept pressing the lever to continue getting pleasure (they pressed the lever thousands of times an hour). You can see a picture of the limbic system at: http://www.psyweb.com/Brain/brainl0.html