Each unit can be one of 4 types of Rarity: Common, Rare, Epic and Legendary
Common Units (displayed as light gray) are your general units. They can be upgraded through 13 ranks.
Rare Units (displayed as Blue) start out stronger than Common units, and often have a special perk. They can be upgraded through 11 ranks. A rank 1 Rare is equivalent to a rank 3 Common unit.
Epic Units (displayed as Purple) start out stronger than Rare units, and usually have a unique spin on a common or rare battle strategy. They can be upgraded through 8 ranks. A rank 1 Epic is equivalent to a rank 6 Common unit.
Legendary Units (displayed as Gold) start out stronger than Epic units, and these units have a unique mechanic. They can be upgraded through 5 ranks. A rank 1 Legendary is equivalent to a rank 9 Common unit.
It is important to promote you units if you're able to because one rank can make the difference between your unit surviving an AoE blast or getting obliterated. Check out your unit info to see what a difference each rank makes!