Major depressive disorder is a medical condition characterized by depressed mood, anhedonia (loss of interest or pleasure in things you normally enjoyed), hopelessness, thoughts of self harm, sleep disturbance (too much or too little), loss of interests, unexplained guilt, depressed mood, low energy level, difficulty with concentration, sudden increase or decrease in appetite, sluggishness/feeling weighted down. People will often feel like they are in an inescapable mood or state and they have difficulty mustering the energy to even begin to address the issue.
Depression is frequently the product of both genetic predisposition to abnormal function and presence of neurotransmitters such as serotonin, dopamine, and norepinephrine. These chemicals are targeted by medications that may help improve neurotransmitter levels. This allows you to experience your environment and emotions in way that is free of potentially dysfunctional neural pathways.