Depression is more than a feeling of sadness; it affects mood in a variety of ways, changes your thoughts, lowers your ability to concentrate or make decisions, and impacts your behaviour so that you stop doing things that used to bring you a sense of joy or accomplishment. 

Some Common Symptoms

• Feeling sad, down, exhausted, angry, bitter, fed up
• Withdrawing from other people and activities
• Sleeping more, or having trouble sleeping
• Having no energy, feeling sluggish
• Feeling helpless and hopeless
• Eating more or less than usual
• Everything feels like it takes too much effort
• Feeling guilty
• Hygiene and other self-care is hard to keep up with
• Thoughts like "Why bother?", "I can't do anything," "This is pointless", etc. 
• Self-harm such as using substances to avoid things or self-medicate your symptoms, cutting or burning yourself, only eating junk food, etc. 
• Suicidal thoughts

Cognitive Behavioural Therapy is one of the effective methods of treating depression. Therapy is often used in addition to medication prescribed by your doctor or psychiatrist, but many people also try therapy before trying medication, or instead of taking medication.