A phaser rifle is the Star Trek Federation's phaser-based equivalent of a modern sub-machinegun or assault rifle. Several different styles have appeared in the course of the series. They are presumably more accurate over long distances than a hand phaser, and they probably have larger power supplies.
Much like hand phasers, Phaser III is an ergonomic disaster that lacks proper sighting features and safety devices like a trigger guard.
Phaser rifles made during the Dominion War have trigger guards and scopes, features that previous models lacked. During the war emergency, Starfleet presumably did not have the luxury of sending every recruit through years of Academy training, prompting them to design weapons that were easier to use.