The Infiltrator is an Ascendancy Class for the Shadow Class, a DEX / INT hybrid class that starts on the upper right of the Passive Tree, similar to the Monk.
GGG has yet to reveal specifics about the Infiltrator, and the other Shadow Ascendancy, Assassin. Furthermore, the Infiltrator is not an Ascendancy from PoE 1, so there is no reference to predict what may come.
Based on the name, we could expect the Infiltrator to make use of the new Stealth mechanic, but that is simply a prediction. GGG will soon reveal more details for the Infiltrator, so check back in the future to learn more.