Depression is personal to everyone. It is more than simply feeling unhappy or fed up for an amount of time, it is a state of low mood and sometimes an aversion to activity. It can affect a person's thoughts, behaviour, motivation, feelings and sense of well-being.
It can manifest as sadness, difficulty in thinking and concentration, a significant increase or decrease in appetite and time spent sleeping. People experiencing depression may have feelings of dejection, hopelessness and sometimes, suicidal thoughts.
Depression can be short term or long term, it can come and go. It is not a sign of weakness or something you can "snap out of" by "pulling yourself together". It is a real health issue and has very real symptoms.