A Blaster, along with the ray gun, is a canonical example of a science fiction weapon. Indeed the two terms could be considered synonymous, with ray gun being the older term.
In science fiction literature or films, blasters are generally portrayed as hand held directed energy weapons that can fire a beam of energy at an opponent sufficient to kill them. The blaster is often a replacement for guns and may vary from a small hand-held weapon to larger, and presumably more powerful, vehicle and ship or spacecraft mounted weapons.
The precise operating technology of a blaster is generally not explained in too much detail the literature. However some form of laser, plasma or particle beam technology is generally apparent from the context.
In The Apocalypse Troll, David Weber described his version of a blaster as "... a capacitor-fed energy weapon which projects a pulse of plasma at the target. "
In many cases the weapons are not even called a blaster but by some other name.