A power fist is a type of weapon that appears in every incarnation of the Fallout universe released thus far; one of very few weapons to claim such status. In all of its incarnations, it appears as some variation of glove or armored gauntlet, with a powered ram or similar device which improves the impact of the wielder's Unarmed strikes.
The pneumatic power fist (referred to in-game simply as a power fist) is an Unarmed-class weapon which appears in Fallout 3, Fallout: New Vegas and Fallout 4. It appears as an armored gauntlet, like that of a suit of power armor. However, it appears to use a pneumatic ram instead of kinetic energy devices, making it function in a manner closer to that of the impact gloves. (see below)
Impact gloves are an Unarmed class of weapon exclusive to Fallout Tactics. They appear to be a leather glove with a pneumatic piston bolted to the back of the hand, which extends on contact with the enemy for additional striking force. While not, strictly-speaking, a power fist, it is included as it functions in a more or less similar fashion. Impact gloves do not require ammunition.
The mega electric power fist is an enhanced version of the electric power fist that appears in Fallout: Brotherhood of Steel. It uses an internal power source, negating the need for power cells. The impact of the electric power fist deals damage similar to a raygun shot.