There is a great difference between sadness, brief unhappiness, and depression. Sadness occurs when one is disappointed or unhappy with a situation. Brief periods of unhappiness occur after losses. Depression, however, is another matter all together.
Depression has many forms. There is a brief or normal and reactive depression. When we have lost a job or a position or a relationship, we can often respond with a brief and reactive and appropriate depression. During this time we are upset, there may be some changes in our general habits, but most people pass through this and pull themselves back together. Mild symptoms are often associated with depression. These may be a mild decrease in appetite, loss in weight, slight lowering of self esteem, an uneven sleep pattern for a while, increased rumination, and low energy level. Again with the vast majority of individuals, these pass without serious repercussions.
     However, there are times when the depression posses a greater and more serious threat to the integrity in the life of an individual. When the depression is significant and appears to last for more than at least two weeks, this is something that should be taken note of. Sometimes with a significant depression there is a serious sleep interruption. This might be that the person experiences difficulty falling asleep for more than an hour at a time. There may be mini awakenings at night. There is sometimes difficulty getting out of bed in the morning and getting to classes. In some individuals there is the phenomenon of hypersomnia, which is excessive sleep. This would be gauged by the need for more than ten hours of sleep. When there is an appetite loss of ten or more pounds, this is also a significant factor. In cases of significant depression these is a decrease in concentration and periods of time in which a person can feel unfocussed. There is often associated this a loss of motivation for school work or other forms of work. This is associated with a sense of fatigue and heaviness. Others may a notice a sense of social withdrawal and isolation by the individual accompanied by sometimes increased irritability. In some extreme cases there is even what is referred to as "psychomotor retardation". This is when the person literally begins to physically move slower and seems to be experiencing a significant decrease in their capacity to move and conduct themselves through the day. Also with depression that is significant there are increased feelings of guilt, sense of uselessness, and loss of pleasure or anhedonia. These are all signs that the depression has moved beyond a mild or moderate stage into something more serious and deserves the attention of those around them.
     The forms of serious depression however are many. Some of these are reactive to a situation. This includes a major depressive episode. A person may require to be removed briefly from the life context in order to overcome this depression. Some depression, however, appear to be cyclical to the persons personality style. In other words, there is a condition called cyclothymic disorder. This is where a person literally has significant ups and downs in their personality during the year. Another condition is referred to as bipolar disorder. There are two forms of this. It is essentially where a person's moods are significantly altered that it requires attention. There are several other mood disorders that are associated with depression. There is a form of depression also associated with personality "style" that is deferred to as dysthemia. This refers to a person who may feel themselves to be low in energy and mildly depressed for long periods of time without any clearly stimulating factors. A variation on this is SAD or seasonal affective disorder. However, seasonal affective disorder occurs primarily on the wintertime and is significantly altered by the change of seasons. Seasonal affective disorder is also know to respond to different forms of light therapy. Many forms of depression also respond well to brief medication.
     A form of depression that is unique to women is referred to as post-partum depression. Sometimes after the birth of a child, there is a significant depression on the part of the woman for several months. This is related to biological and hormonal changes as opposed to primarily psychological factors. Clearly psychological factors are involved, but it is primarily a medical disturbance.
     Another form of serious depression that is normal is the bereavement response. This is when someone very close to us, usually in our family relationships, but also in our other intimate circumstances dies. We have a painful but normal and prolonged grieving process. In most individuals this generally lasts about a year. Many cultures and societies naturally address this with various rituals and patterns of response to help the individual move through these difficult times. This again is a normal response. When it moves beyond a normal response to one that is a clinical response, it is referred to as melancholia.
     There are forms of depression that are brief and reactive that are initiated not by the personality but by substances. These substance induced mood disorders are many. Also occasionally there is an intense form of depression referred to as psychotic depression. These can occur with intense manic episodes and with other major depressive episodes.
     It is important to know when to seek help for one's self and others. When the depression occurs for more than a month, it is important to address that depression. Also when the depression appears to be continuous, it is important to address it in yourself or to others. If significant changes in habits such as sleep, appetite, concentration, mood, subjective negativity and isolation are all signs that the depression should be taken seriously. In extreme situations suicidal ideation, whether it be recurrent or fleeting, can attend depressive episodes. These brief suicidal thoughts can occur in one's self and sometimes be observed in others. When they are fleeting, they may occur to a person for the first time and actually be quite frightening for an individual. Others however, have experienced suicidal ideation off and on many times and are not as frightened by them. However in either case they should be taken seriously. When there is a sense of finality and a sense of severing of ties, of unrelenting guilt, remorse and feeling of worthlessness, this is when depression is associated with suicidal ideation that should and must be taken seriously. Verbalized statements of self injury to self or others are also to be taken quite seriously. When messages are left to others in various ways symbolic or other, this is a sign that the person's behavior should be taken quite seriously. This is not the time to abandon a friend. It is better in circumstances like this to be nosy and intrusive with someone that to abandon them. Also when there is an increased fixation on (exit) options, this is sign that the person is entertaining destructive thoughts beyond the fleeting level.
     It is extremely important to take such subjective and behavioral signs seriously. When this occurs to one's self, even if it's for the first time, one should seek help or if it occurs with someone else, one should gently urge the others to seek help. One should be aware of the emergencies operation. Here at the university, you have available the University Health Services Mental Health Division, which is a 24 hour on-call service. There is also Emergency Services in Northampton. There is also the police and other agencies.
     Help for someone with a significant depression that may be associated with a suicidal ideation can take many forms. Sometimes brief counseling or even longer term therapy is important after there is an assessment by an appropriate clinician. Sometimes brief counseling may be associated with appropriate medication. There are a number of very effective medications that significantly decrease the pressure that is created subjectively with depression and suicidal ideation. Sometimes leaving school or one's job can actually be a positive step in decreasing one's immediate life stresses. Under certain circumstances hospitalization, voluntary or involuntary, is the path of choice. Again it is more important to be persistent and even nag the person rather than to isolate them and leave them alone. Better safe than sorry. Compassion is significantly less expensive than grief.

References

Diagnostic and Statistical Manual of Mental Disorders (DSM IV), 4th edition, American Psychiatric Association

Alfred Friedman, MD, Harold Kaplan, MD, & Benjamin Sadock, MD, Modern Synopsis of Psychiatry II