The "Tier" gear is what you get from endgame dungeons. For example:
Molten Core drops mostly T1, although you can get the T2 pants (T2 head comes from Ony) like Giantstalkers
Blackwing Lair drops T2, like Dragonstalker
Nax drops items to turn in for T3 gear, like Cryptstalker
T0 is the blue sets (like Beaststalker) from places like UBRS, Scholo, Strat, ect. T0.5 is a quest line to upgrade the T0 gear, and you have to actually have the T0 piece to upgrade also.
Those are the main tiers, some people have "tiers" for other things... like they may call the AQ40 set T2.5 or something. Note that the current tiers will be about on par with between level 62-65 blue items according to BC beta testers. T3 is supposed to be about a level 65 blue, and T1 like 62 blue or maybe green.