There is a great writeup here, but here is my description of the difference.
This is typical depression for me:
Sometimes you wake up and it takes a moment to settle in, like a fog. Other days you wake up and feel as if you’ve been buried in it. It’s the thickness of the air and the world that makes it hard to move. Every effort is exhausting. But there is no sleep to rejuvenate you, and by the time it’s proper sleeping time you won’t be able to.
It’s not a sadness or a despair, nothing that satisfying. It’s a grey plasma that you have to push through to get to the normal goals everyone else takes for granted. Sitting. Showering. Talking. Pretending you’re fine. Luckily, the grey takes away the desire to do most things, so you don’t have to push as hard. But to everyone else it looks like calmness. Like quiet stillness. Or laziness. But there is something in the background yelling at you to keep moving, that where you are or what you are doing isn’t right, but that something never tells you where you should be. It never tells you what’s right, only wrong. No sitting this one out.
If you live in the thick grey world for too long, it begins to look normal to everyone else. But every moment that you are stuck in the muddling mess is like the first and last moment. Every moment is like every single moment. You cannot see out of it, it’s not a cloud following you around, it’s the world your way, and nobody else seems to be caught by it.
Eventually it will pass. Some wind will blow and it will clear away. Some morning you will wake up and it’ll be receding slowly. Slowly enough that you can’t see it move. But eventually you don’t feel it anymore. But the gray takes away the satisfaction of knowing it’s gone. Just like you can’t see out of the thickness, you can’t see into it. When it’s gone you won’t be able to remember how it felt or didn’t feel. You won’t be able to even believe it happens as intensely as it doesn’t. So when it’s gone it’s business as usual. And nobody congratulates you for clearing it, because they could never see it. And to be certain, the thickness never mentions it. The thickness just sits there and waits for the next day it finds you.
So that’s what I used to call the “mental flu” and I’d have it for a few days or weeks every couple months, sometimes only a few times a year. Other times more frequently. But it always passes.
For me, this major depressive episode (my first) has some similarities. The difference though is that it’s not like a dream. You don’t wake up from it, and it invades your every moment. The fog is different – the world is still in color, the thickness is still there – but it’s like a waking dream. It’s as if my normal depression was a dream about cold and ice, but you know you’re in a dream, and somehow you can sense that you might wake up and it’ll be all over. But this major depressive episode is like you woke up and you’re still freezing to death, and there’s no end in sight. It doesn’t get better, and it just always hurts. The moments you get warmth you still feel cold.
If depression were a dip in your emotions that you fall in and out of, the major is a hole you fall into and the walls are too steep to see a way to climb.
But that’s just me.