'Test' is a full server environment issolated from the rest of the servers. It is in all intensive purposes a special rules server that changes are immediately pushed to for the purpose of testing their impact before it is sent out to the masses. You roll on test if you want to plan and end-to-end experience on a server that sees the content first and actively participates in refining it with the development team. This means you start at level 1 with no gear and must find your way as you first did when you started EQ2. The only real bonus on this server is the permanent XP bonus.
'Test_copy' is a server that also runs much the same build but this server allows players from most other realms to copy already existing toons and gear to this server to 'unit test' specific new features being brought in. This allows you to quickly test something you are interested in without needing to roll a new toon, level it up, gear it, and then test what you are interested in.
I think you'll find 'Test' is largely populated with folks that participate heavily with, or employed by fan sites. While this is a generalization, the test server allows them to see content first and get their write-ups done, and all other stuffs prior to it going live. Since you have to build up your toon there, it isn't a flavor of the update environment but a small steady base of players.
You'll find that 'Test' has little grouping and nearly no raiding by the standards we would commonly define them on live servers. In my experience its heavily populated with solo questers, crafters, and decorators and not mechanics focused folks. BTW, this is by no means a dig, its just painting the landscape as I see it.
And, FWIW, I've not spent alot of time personally on test, I've just been friends with those that have. So as much of this is 2nd hand as it is 1st hand.
Hope that helps.