Just as I said before 'Depression has many faces' - & those 'faces' are different types of it: 

1. MAJOR DEPRESSION ( also called 'major depressive disorder', 'unipolar depression' or just 'depression' ).

Its most visible symptoms are: very low mood, loss of interest in any usual activities, inability to feel pleasure & happiness. tiredness, helplessness, ( & suicidal thoughts in the most severe form ). Those symptoms are long lasting & last at least 2 weeks - but they can last many months as well. They interfere with social aspects of person's life & work. 

There are 3 forms of this kind of depression taking the symptoms intensity into consideration - & it can be defined as:

- mild ( the least dangerous but still needs treatment )

 - moderate 

- severe ( this one is the most dangerous because people with severe depression usually take a suicidal attempt - sometimes many attempts, that may finally turn out successful - that's why it's so important for them to get the right treatment before things get that bad ).

2. CYCLOTHYMIC DISORDER ( also called 'cyclothymia' )

Is a more chronic but milder form of bipolar disorder.

Its most noticeable symptoms are numerous mood swings with hypomanic ( euforia with or without irritable mood - but less severe than 'full mania' ) symptoms that don't fit the criteria for a hypomanic episode, mixed with periods of mild or moderate symptoms that are not as serious as 'full major depressive episode'.

A person with cyclothymia may feel 'stable at a baseline' but experience mood shifts from high euforic to his mood going down to symptoms similar to moderate depression. 

It's less frequent than bipolar disorder, but is becoming more common nowadays.

3. DYSTHYMIC DISORDER ( from Greek: 'Bad state of mind' 

- also called 'dysthymia or 'chronic depression' )

Has the same symptoms as depression, not as severe - but longer lasting than major depression. It lasts for at least 2 years! ( for an adult person ) & 1 year for a child or adolescent. So if it lasts so long - the person with dysthymia may not even know that he is ill - he & his environment may just think that 'it's a part of his character' - & he may not even be diagnosed because they wouldn't think that he needs any diagnosis, they all may think that 'he's just like that'. - But with the right diagnosis such a person may be cured & it would save him at least a year of life of living in a conatant bad mood! 

* What's interesting - Dysthymia may occur together with other disorder - such as major depression - & then we say that such a person has 'Double Depression'! ( as if one was not enough, right? ) 

4. PSYCHOTIC DEPRESSION

Is when a depressed person loses touch with reality & experiences hallucinations ( can see, hear, taste, feel things that are not there ) or delusions ( false beliefs that are not shared by others ). People suffering from Psychotic Depression may suddenly start believing that they are evil or that someone is watching or following them wherever they go or whatever they do. They can become Paranoid believing that 'everyone is against them' or that 'any bad event happening around them was Their Fault'! - So this kind of depression Really Makes Their Lives Difficult - cause they are Blaming Themselves for All The Tragedies in the World - & live in constant fear of the 'non existing person who always watches & follows them'. 

5. CATATONIC DEPRESSION 

This kind of depression makes a person who suffers from it speechless & motionless for an extended period ( so they can not say a word & just sit or lie in one place, not changing their position & not moving for hours. & why don't they move or talk for so long? - cause They are just 'too weak & tired for that' ). Catatonic depression makes such a 'simple everyday activity' as getting up from your bed in the morning almost impossible for them. The other symptoms are low mood, feeling of worlhlessness, problems with eating - similar to major depression. 

( & in their case staying motionless is not a 'Mannequin Challenge' & fun - it's an Illness! - They need help! - Cause it's not funny for them at all. They don't stay motionless 'because they want to' - they stay motionless because they lost their whole energy to move! - &they need help to get up, get dressed or go out of the room as if they were 'disabled' even if their all limbs are ok ). 

6. SEASONAL AFFECTIVE DISORDER ( with an accurate short form 'SAD' - also called 'winter depression' )

This mood disorder usually starts when days get shorter & there isn't enough sunlight. The person who suffers with 'SAD' can have depression symptoms when Winter starts & feel better when the Sping comes & days get longer again. A person with 'SAD' has visibly less energy, low mood & can sleep too much & overeat ( so it's usually 'the other kind of sleeping & eating disorder' than in major depression - cause people with major depression most often can't eat or sleep ).