This 3D animation from The Human Atlas explains depression.

The brain is composed of millions of interconnecting nerve cells called neurons. In order for a person to think, move, or feel, these neurons must communicate with one another. They do so by sending and receiving chemical messengers called neurotransmitters.

When a neurotransmitter is released from a neuron, it crosses a cleft, or synapse, and binds to a receptor on another neuron; thus the signal is passed on.

Unlike the blues, sadness, or even grief, depression is a condition in which a person feels an overwhelming and debilitating unhappiness. People who are depressed may have trouble thinking clearly and be unable to perform normal functions. They may be uninterested in eating and be unable to sleep, or they may engage in these activities excessively. Persons with depression may even have thoughts of suicide.

Although depression can be triggered by an emotional event in a person's life, a decrease in the levels of one of the neurotransmitters, serotonin, has been linked in the biology of depression.

There are many forms of depression. Therefore it is important for a person who may be depressed to see a doctor who can accurately diagnosis the depression and prescribe appropriate therapy.