Depression isn't just a bad mood. It's not even a question of feeling bad. Sometimes, that's not even the form it takes.
Depression has symptoms that are physical as well as mental. Movement is sluggish, response is slow. I took an IQ test once in a depressive state and scored twenty points lower than I test normally. Physically, in an aggravated state, I'm fatigued and often sick, sometimes even a bit dizzy. I went to the doctor for dizziness and headaches three times before he told me that there wasn't anything wrong with me in a neurological sense--all the symptoms were coming from depression. This didn't make me feel any better, as I felt I should have known this and not bothered a medical professional with stupid phantoms from my own messed up brain... it wasn't like I didn't know this could be a depressive symptom. I took AbPsych, for heaven's sake...
And so on.
The worst part, for me, is my imagination turning on me. It doesn't go away. It just starts filling in little things for me, like, "You don't have friends in town because you're unlikeable, and as soon as you leave the room, everyone heaves a sigh of relief and gets down to the business of having a life." And of course, "If the people who talk to you online met you, they'd think you were a fat, boring lump." Or maybe it would go on a medical kick... this is actually the most common lately. Every time there's a commercial for one of those new prescription drugs--you know, the ones that list vague symptoms so that everyone will harass his or her doctor to put them on this new pill--I start doing an inventory in my head and deciding that I'm probably dying of some horrible thing. Now, I can recognize this intellectually as stupid--thank heaven, I'm a T rather than an F--but that just makes me start kicking myself about being a total goose, which makes the mood part worse.
It also gets tied up with anxiety, and--very unfun symptom--obsessive-compulsive behavior. For me, it's checking the locks on the doors (because if I don't, someone will break in as soon as I'm not looking and all my roommate's things will be stolen and it will be my fault) and a checking routine on things that really aren't fire hazards but which I convince myself will burn the house to a cinder if I don't check. At the worst of it, I unplugged my computer, my radio, and my desk lamp every day before going to work, because I'd somehow come to the conclusion that they were overheating the circuits. I did get one bit of fun out of my OC behavior, though--I went to visit an old friend, who turned out not only to also be suffering from it, but to have the same fixations. We laughed at each other about checking car door locks quite a lot, and laughing is very good.
Then there's the insomnia, which is self-defeating, because being tired only makes it worse. Then that tends to be followed up by bouts of sleeping for ten to fourteen hours. Which isn't healthy.
Another problem is anhedonia. Deciding to do things that will cheer me up doesn't work, because the things I normally enjoy don't do anything for me.
I've tried a couple of different medications; Lexapro works best, but I tend to forget to take it. The most effective strategy I have doesn't exactly make me happy, but it helps get through the worst of it (usually)--I just assert my mind over the weird little chemicals my brain is producing. It's actually something I picked up in my reading about OCD--I had to stop the unplugging. The phrase is kind of a mnemonic: It's not me, it's my OCD. The book is called Brain Lock, and it deals with asserting control and creating new neural pathways by breaking out of the obsessive thinking. It occurred to me that this could be at least somewhat helpful with the imaginary catastrophes of depression as well--"This isn't real, this is a phantom." "This emotion isn't caused by anything; coast above it."
It doesn't always work, but it does often enough to get through day to day most of the time.
Anyway, that's my inside knowledge of depression and how I deal with it. Don't know if it's helpful to anyone.