OK, can someone explain to me what the point is in buying and selling these "trading cards" you get in Steam?  And WTF are Gems?

I can see why you'd want to sell a lot of trading cards at once in an attempt to make a couple of quid, but why would someone want to buy them in the first place?  Or do you sell them direct to Steam? 

one collects cards to get achievement cubeS obviously

one sells cards to get MONEY obviously

If you gather enough you receive a pass to the Half-Life 3 beta.

You can use them to craft achievement cubes and "level up" your account which has some advantages like every 10 levels I think you get a higher chance of receiving "Booster Packs", higher chance of receiving foil cards (worth more $$$), more friends list slots, extra showcases for your profile, etc

If you don't care about any of that you can just sell them and it's basically free money

Gems can be used to buy "Booster Packs" (which give you 2 or 3 random trading cards) or you can pack 1000 of them into a sack and sell them about $0.55/sack currently

it's all just another e-dick competition to make people buy more steam games

and the sad thing is it works so incredibly well

Yeah good thing is, those who don't give a crap about it also profit!