Depression is a disorder comprising of a group of symptoms including low mood, loss of interest in leisure activities, low energy levels and concentration, poor sleep, low self worth, feelings of guilt, negative thought processes among others.
It can sometimes be used and confused with sadness, feelings of low etc., felt by everyone at one time or another, usually as a reaction to life events or stress. These normal emotions are time limited and do not require treatment.
Depression of varying severity is common, affecting between 30-50% of us over our lifetime. Such a high prevalence of depression would mean that it will affect someone in your family or amongst your friends at some point in their lives, at any age. Half the people suffer depression only once in their lifetime but unfortunately the other half would have recurrent episodes.