The Assassin 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 Assassin, and the other Shadow Ascendancy, Infiltrator. However, based on PoE 1, we can guess that the Assassin will have some theming surrounding the following:
- Critical Strike
- Power Charges
- Poision/Ailments
In general, the Assassin is a glass cannon type Ascendancy Class in PoE 1, using careful defensive layering to maintain survivability. However, PoE 2 could switch things up. We'll just have to wait and see.