Serotonin is a neurotransmitter linked to depression, especially medium or mild intensity depression. Serotonin levels drop during depression, so most antidepressants are build in the most selective manner. There are pills that react with presynaptic 5 HT 1A receptors. They prevent the reuptake of serotonin by the presynaptic neuron, thus maintaining higher levels of 5-HT in the synapse. SSRI treatment (Selective serotonin reuptake inhibitors) is the first step in depression treatment. Here we have: Fluoxetine (our classic Prozac), Sertaline (Zoloft, another classic), Paroxetine, Fluvoxamine and Citalopram. Other medication reacts with postsynaptic receptors, and others on both groups. Their success is influenced by how they managed to increase the serotonin levels necessary for nerve impulse.
Serotonin and types of depression
When it comes to depression, the type that starts with low serotonin levels is an anxious depression with psychomotor agitation, frequent crying, irritability, irascibility, and a patient that will still struggle to get out of this state. There is no pathology at the cognitive level (perhaps some attention and concentration difficulties).
When we discuss about major depression, we must add that it is associated, besides low serotonin levels, with low noradrenaline levels. Some of the symptoms are: psychomotor inhibition, severe cognitive deficiencies, weight loss, oversleeping, even athymhormia.
Psychotic depression (appears in 15% of people who suffer from major depression). Symptoms include: auditory and visual hallucinations and delusional thinking (mood congruent delusions like paranoid delusions, delusions of guilt, delusions of persecution), thoughts of death, etc. As for neurotransmitters, besides serotonin and noradrenaline issues, there is a high increase of dopamine, which can be treated with narcoleptics or typical antipsychotics.
More on serotonin:
Picture taken from here.