Depression is a state of mood where people usually feel restless, worthless, listless, afraid that things aren't going to be better. There are many factors that can be the cause of depression, ranging from life events to various health conditions that aren't related to what is happening to us in our daily lives (parasites, viruses, bacteria, tumors,...) . For all of this there is treatment. Depression caused by some alment is easily treatable, and can be gone when the cause is removed, whereas depression caused by some external factors takes longer time to be completely gone. It might require years of therapy and support from friends and family, but with the right care, and some things the person who is suffering from depression can do to make their condition better, the quality of their life can immensely improve.
However, if even that doesn't help, and the person is having suicidal feelings and thoughts, please refer to this page for more solutions.