Sentinels have a wide array of abilities; they can fly, shoot various weapons from different parts of their bodies, detect mutants at long range; some of them can even change form and re-assemble after being destroyed. Most dangerous of all, some varients have the ability to learn from the attacks they face and create defenses against them which hampers the opponents' chances of defeating them.
The most famous sentinels are The Master Mold and Nimrod.