There are currently 3 tiers of troops, often referred to as T1, T2, T3s

Tier 1 = Catsith

Tier 2 = Gryphon

Tier 3 splits into 3 type of troops, each good at one specific thing (attacking in PvP, defending in PvP and attacking monsters + carry capacity)

Troop research can further upgrade their power (attacking = strength and defending = toughness) along with other upgrades like march speed