There are no real "tiers" of equipment. Instead there are rough groups of armor that match visually.
If all of your items are from that given group, they will match. However, this does not determine their stats.
From Blizzard Community Manager Bashiok:
We separate out 'item tiers' for their visual appearance first and foremost. We have a number of "armor sets" (18?) in the game that match visually, and so we call them 'tiers' because there's no real better way to say it. When all of your items are from that 'tier', you'll match. That doesn't directly determine their stats, though.
They're spread out in a logical progression, and items get more awesome looking as you go further in the game. Ipso facto the higher the visual tier the better chance it's a more powerful item, but it doesn't preclude the possibility of it being worse. You can absolutely find an item from a visual tier back in Hell difficulty that's better than an item using a later visual tier in Inferno.
It's best to just expunge the term 'tier' from your minds, because it really has nothing to do with any kind of stair step or item upgrade approach. It's just models and textures.