Most people go through periods of feeling down, but when you're depressed you feel persistently sad for weeks or months, rather than just a few days. Some people think depression is trivial and not a genuine health condition. They're wrong – it is a real illness with real symptoms. Depression is not a sign of weakness or something you can "snap out of" by "pulling yourself together".
As well as a very low mood there are a range of other symptoms that go along with it. For example, feeling hopeless, sometimes feeling suicidal, and also having biological symptoms, such as loss of appetite, difficulty sleeping, and the loss of libido. The good news is that with the right treatment and support, most people with depression can make a full recovery.