There are a number of different types of depression. First, there are the different degrees of depression, generally categorized into mild, moderate, severe and chronic. It can also have different features: Psychotic, Catatonic, Melancholic, and Atypical. Then there are the different frequencies: if you have ony had one major depressive episode it is "Single Episode" (creative terming, eh?), or if you have had multiple episodes you are identified as to whether you are currently in a depressive episode, sort of depressed ("In Partial Remission"), or not depressed ("In Full Remission"). And this is just for generalized, sterotypical depression. Below is detail on some specific depressive disorders.
Seasonal Affective Disorder
Commonly abbreiviated "SAD," (the name is very appropriate, eh?) this is one of the most common forms of depression (more than 10 million people, out of the 18 million diagnosed with depression in America, have SAD; of all the people who have SAD approximately 75% are women, and SAD commonly begins to show up in the mid-thirties), although it often goes undiagnosed because goes away after just a couple months, even though it just comes back year after year after year, since by the time someone has started to notice changes in the person in question the depression has already begun to dissipate.
SAD is essentially an extreme form of what almost everyone experiences during the winter, extended periods of cloudy, overcast, dark weather, or if one works in a place with little natural light (in which case SAD can actually develop at any time of the year, and not go away~ in which case it is essentially indistinguishable from normal depression except that its main cause is an overdose of melatonin instead of the host of other possible causes of other incarnations of depression). Many people will notice tiredness, weight gain and a difficulty to wake up and just "the blues" in general, and these symptoms are caused by the lack of light (low light causes the body to produce melatonin, whose normal function is to help you sleep) that occurs in the winter in temperate regions of the globe. SAD develops in individuals with a heightened sensitivity to light and lack thereof, the symptoms that begin in the autumn, reach their climax in the winter and go away again in the spring are mostly just extreme versions of normal low-light symptoms~ a lack of energy, increased need for sleep, a craving for sweets, weight gain and all the symptoms associated with normal depression. Individuals also often experience episodes of "mania" during the summer.
People with SAD respond favorably to light therapy (exposure to a "light box" for a certain period of time at a given distance (both of which vary with the symptoms) regularly, or medication can be used in some cases to alleviate melatonin levels. However, sometimes the condition is so extreme that it refuses to respond to this. Often spending time outside is more efficient than the light box, though, and sometimes the individual, depending on their sensitivity, will be forced to move to a lower latitude if no other treatment will work. Also, eating a balanced diet has been shown to have a positive effect on the symptoms of SAD (although you should eat healthily no matter what!). But the most effective way to ensure that you do not develop SAD is to spend some time outdoors, soakin' up the good ol' sun (just don't let yourself get skin cancer or anything like that, wot wot? Guess you can't win...).
Bipolar disorder is essentially where the patient has periods of extremely top-of-the-world moods (known as "maniac episodes"), and extremely depressed moods (known as "depressive episodes"). All people have differing moods- we all have emotions, don't we? But Bipolar is a very devastating disorder~ some example statistics include 3 times the rate of divorce, twice as high incidence of mid-life crisis, and an elevated suicide rate.
Maniac episodes are characterized by feelings of happiness and total competency, which by themselves are not bad, but also it causes easy irritation, arrogance, fast speech, easily distracted, inability to sleep, a need for pleasurable activies without regard to the risk associated, way too much energy (is that possible? **sUgAr**!!!!!), and no concentration whatsoever. The depressive episodes are the same as depression, the only way that a depressive episode of Bipolar Disorder is differentiated from that of normal depression is that they are accompianed by Maniac Episodes.
Depression in Children and Adolescents
Depression in children is not as well-studied or documented as depression in adults is; the assumption commonly made that children cannot become depressed is most likely to blame.
Among those who do accept the existence of depression in kids (and teenagers), there is considerable disagreement over what form it takes. Although it is quite obvious that children can be sad or "blue," it is easily argued that full-fledged depression such as is present in adults cannot occur.
The earliest views on childhood depression (back in the dark ages when psychology was still a fledgling science out on the fringes) took the stereotypical stance, that depression as a clinical disorder in children was simply impossible. Later other interpretations started emerging, although the idea of kids being severely depressed remains laughable to the general public.
The leading view today of depression in children and adolescents is that the possibility for depressive episodes occurring is closely intertwined with a number of factors, including prior experience, the environment, genetics, psychological maturity, and perceptual and cognitive skills. Thus, it is possible for kids to develop depression but not anywhere near as likely as adults.
Then there is "Masked Depression," which states that depression does pop up in children, but its not the same as the adult depression, at least symptomatically. Depression of kids shows up in symptoms like temper tantrums, hyperactivity, disobedience, extreme/large numbers of phobias, separation anxiety and under achievement. The problem with this view is that it is much too all-encompassing, that these are "symptoms" that almost all children exhibit at some point or another just while normally growing up, and even in their extreme forms are often symptoms of very different childhood disorders. And this inevitably led some to the conclusion that depression occurs simply as a part of growing up, which was generally rejected. But this idea of depressive symptoms being just part of normal development states that depression emerges in kids simply as part of normal development and dissipates over time. This is easily taken down by arguing that although there are symptoms of depression present, that does not mean that there is true, clinical depression.
But even if it was full-fledged depression that was common among children, simply because it is common does not mean it can simply be written off. There is also a suspected connection between childhood depression and adulthood depression~ and if having depression as a kid means you are more likely to become depressed as an adult (which would not be suprising, since every depressive episode you have as an adult puts you at higher and higher risk for having another), then childhood depression is definately something that needs to be documented. The current issue is: how?