An antidepressant is any one of a number of drugs that augment the action of noradrenaline and/or serotonin in the brain. As a result, the brain is mantained "excited"; in depression, there is a lack of noradrenergic and/or serotoninergic transmission. Antidepressants are currently seen as more effective than mere psychological intervention in treating depression.
Some antidepressants take up to 3 weeks for the therapeutic effect to occur; events of suicide during this period have led to the false belief of suicide being a side effect of antidepressants. Patients are put on Benzodiazepines during this time in which the drug is inactive. In addition, sudden cessation of the drug can often have catastrophic consequences, even if the patient is put on a different type of antidepressant.
Antidepressants are one of the most commonly prescribed drugs in the United States. Luckily, it appears that they have few or no side effects for people who are not actually suffering from depression.
James Wilson is known to be taking antidepressants. In the episode Resignation, Wilson deliberately doses House with antidepressants to demonstrate that most of House's misery appears to be from clinical depression.