In Wings of Liberty, the aberration appears in missions where infested terrans are prominent. It is very powerful with a lot of HP and armor, and a powerful melee attack. However, it can be brought down easily enough with concentrated fire.
Heart of the Swarm
In Heart of the Swarm, the aberration has less HP than in Wings of Liberty, but has a powerful buff to its damage against armored targets. It also has the passive ability "Towering" that lets it walk over other units similar to the colossus, but without vulnerability to anti-air attacks. While the in-game text says the aberration can walk over small units, the Massive and Light unit types have nothing to do with this ability. The only zerg units the aberration cannot walk over are infestors, swarm hosts, ultralisks, and Sarah Kerrigan. Aberrations are available to build upon playing the mission "Old Soldiers."
The aberration is an excellent support attacker for ranged units like hydralisks and roaches; when the ranged units stop to fire at enemies, the aberration can still walk over them to get to its targets, and moves as quickly as a zergling to get there. They are powerful throughout the game since many terran enemies are of the armored type. In the late game, aberrations offer complementary skills to the ultralisk; while ultralisks are effective against clusters of small units, aberrations are better against small numbers of armored units. The ultralisk also allows variable upgrades and its noxious or torrasque forms to enhance its combat potential, while the aberration is static and has no potential upgrades.
In Co-op Missions, aberrations are exclusive to Zagara, and require an evolution chamber and lair. Apart from having an armor value of 0, they are statistically identical to their Heart of the Swarm appearance, functioning as anti-armor attackers.
Aberrations were previously known as centaurs. It had 'baneling bubbles' on its back and was originally intended to be a cross between a baneling and infested colonist, exploding like the former. The final aberration has a different combat role; its explosion upon death is purely cosmetic.