According to Wookiepedia [blasters][1] are :

>a ranged weapon that fired bursts of [particle beam][2] energy called blaster bolts from a replaceable power pack.

On that same page for blasters, under Blast Mechanics:

>Instead of firing a coherent beam of light like the archaic laser, the blaster fired a compressed, focused, high-energy particle-beam that was very destructive, commonly referred to as a "bolt." Generating the bolt relied on two components: a gas cartridge filled with an energy-rich [blaster gas][3] (typically Tibanna) and a power pack. When the blaster was fired, a small amount of gas moved from the cartridge through the Heter valve into the gas conversion enabler chamber, commonly called the "XCiter". In this chamber, the power pack energized the gas, before it passed into the actuating blaster module, where the now extremely high-energy gas was transformed into a compressed beam of intense energy particles, coupled with intense light. The particle beam was then focused through a prismatic crystal or a similar device, which generated a deadly high-energy particle beam, fired from the emitter nozzle as a bolt of glowing energy. The color of the bolt depended on the gas used and the type of focusing device, and could vary from red to blue to orange to white.


  [1]: http://starwars.wikia.com/wiki/Blaster
  [2]: http://starwars.wikia.com/wiki/Particle_beam
  [3]: http://starwars.wikia.com/wiki/Blaster_gas