The Sentinels are special forces soldiers that have had implants placed in their body. Their bones, skin, and muscles are denser and more durable, they can run faster than a humans, and have quicker reaction times and improved eyesight. Some of them have special abilities, like creating kinetic barriers that nullifies Cyclops' optic blast. Their armor (which look like the Sentinels in the comics) are even more durable than their bodies. The armor has its own oxygen supply, is covered with ceramic mind-shielding plates, and the rubber inseams prevents them from being electrocuted. They carry automatic weapons that shoot darts filled with tranqs/x-gene ******ant.
Pretty much in short, they're supposed to be the government's answer to mutant powers.