Depression is characterized by sadness, loss of interest in things once enjoyed, low self-esteem, hopelessness, feelings of guilt, fatigue, difficulty concentrating, anger or irritability, physical complaints (such as stomachaches or headaches), and changes in sleeping or eating patterns. It can also include suicidal thoughts.
There are several theories about what causes depression, so treatment is varied as well. Therapy may include addressing unhelpful thoughts, creating a daily schedule, or working through the specific situation that led to the depression.