Let´s say it this way...
Tribal is way to get some resistance items, suchs as Frostsaber
wich gives Frost and Shadow resistances. Then they can of course make Devilsaur items, rogue/ feral druid item. Suggest for Druids. I think ionly Tribals can make Hide of the Wild, not sure.
Elemental is way to make such items as Stormshroud. Prefer for rogues. Don´t know much about it.
Dragonscale is clearly for Hunter / Shaman use. Some fire resistance giving items for MC, Alot attackpower from Black Dragonscale. They´re bit like few resistance giving items wich have somekind of extra bonus in them. Such as Red Dragonscale Breastplate, 12 FR (i think) and 66 + healing. Hunter-Shaman choice.