Depression has become a more known and better-understood psychiatric disorder in the past few decades. Unfortunately, we use the word all too easily when we casually say 'I'm feeling depressed'. It can vary in severity from mild to very severe, with people unable to function at work or in their private lives when it is at its worst. But in its milder forms it can involve a low and flat mood for several days or weeks, where negative thoughts (about self, the world and others) are common, combined with a lack of energy, excessive or inadequate sleep, increased or decreased appetite, thoughts of suicide (not all the time), a lack of motivation, decreased pleasure in activities you normally enjoy (sports, sex or a hobby) and, occasionally, a lack of libido (but, often people can still get it up and have sex).
Depression in its various forms should not be ignored. Medication is usually helpful for more severe forms of depression, but ongoing psychotherapy is useful and necessary for all forms of depression EVEN when medication is being taken. Often, depression and anxiety can be linked to other problems like substance abuse and addiction, eating disorders (such as compulsive eating) and relationship problems.
Do not leave depression untreated. The more severe depression becomes, the higher the risk of suicide. Suicide is also possible in cases when the depression begins to lift (as energy levels begin to increase). If you or your loved one is living with depression, turn to the resources around you: family, friends, therapists, religious support etc.