How can I begin to describe depression to someone who has never had it?

Describing depression to someone that has never had it is impossible. You cannot understand something you've never experience, you can only imagine. You cannot explain depression, you can however, explain that you need someone.

An absolute desire to get everything over with. You have no motivation and just feel like letting everything drift away. Absolutely no interest in anything, that includes sex, and you just feel like everything is over.

You feel lost in a black hole which seems inescapable, dark, cold and hopeless. No light seems to be able to save you

One of the main things is the dopamine and serotonin is not balanced in your brain. Your emotions become more intense, especially sadness and anger if someone is depressed, and it's harder to regulate emotions like that.

Many people confuse depression with extreme sensation of sadness. But actually, I think it is feeling nothing, which is actually more worst than being extremely sad.

It's like a dark force always hanging over you and making you feel horrible about anything you say or do. You feel like everything is hopeless and nothing makes you feel happy anymore. You can get out of bed because what's the point. I wouldn't wish it on my worst enemy.

Describe it such as a rainstorm that does not stop, and explain the feelings that Depression brings to you on a daily basis.

I would begin by making it clear that depression isn't about simply being "sad" all the time. Depression, instead, is a treatable mental disorder whose symptoms express themselves in a variety of ways, one of which includes being "sad." I would then go on to describe some of your other symptoms, such as feeling a lack of motivation, feeling like you can't enjoy the things you used to love, and possibly even thoughts/attempts at suicide. It's important to against differentiate depression from normal bouts of sorrow, which we all get. Depression is just as real of a mental disorder as schizophrenia, and should be treated with the same seriousness-- it's not simply "being sad." That said, remind the person that it -is- treatable, and that help is available (through drugs, talk therapy, behavioral modification, etc).