Depression is a common mental disorder that affects around 2 in 3 adults at some point in their life. Depression that is serious enough to need treatment occurs in around 1 in 4 women and 1 in 10 men, and is more common in the LGBT community. It is, itself, defined as a feeling of low mood daily for 2 weeks or more.
The core symptoms are a persistent low mood and/or a loss of interest in pleasure in activities especially in those that you normally enjoy. Other common symptoms are changes in sleeping patterns, changes in appetite, tiredness, slowing of movements, poor concentration and inductiveness, feeling of worthlessness or guilt, recurrent thoughts of death (including thoughts of suicide).
There are many treatment options. If you are worried that you might have depression (not everyone realises that they do) then it is best to see your GP. They will be able to refer you to counselling and may also be able to help stabilize some of the symptoms you are having.