WoW Tank Gear Question...?

My main for the longest time was a Balance Druid but recently I decided I wanted to start Tanking. I made a Warrior and am now at lvl 75. As I've gotten higher and higher though more people have been telling me get "tank gear" and so on. I've got almost all blue plate gear and so on so I'm not too sure what they are referring too. Also, could someone tell me whats the difference between pve and pvp gear. I guess I'm just still confused over this.