Depression is, basically, as much as a disease of the mind, a problem of the soul. Out of all the metaphors and similes I've seen for depression, the most accurate would probably be a dark grey cloud, or fog. Depression dulls edges, shrouds the world, and isolates you from everything around you.

Other good descriptions: a weight around your neck, apathy of the soul, wanting to sleep and not wake up.

On a more advice-like note, depression is, for most people, not something to be dealt with alone. Depression is an illness. It can be diagnosed, treated, and cured. It can be fought. Be strong, and get well.