A MAC in halo, at least the ones on UNSC frigates, are rail guns(a weapon with 2 parallel rails connected to a power source), which fire projectiles by sending an electric charge through a rail, then through the projectile inbetween the 2 rails, and then through the other rail and back to the power source.

This creates something called the Lorentz Force, which causes the projectile to accelerate at a velocity proportional to the length of the rails and the amount of power being sent through them.

Fast forward to 35 seconds in this vid to see the frigates rail gun

http://www.youtube.com/watch?v=wg__WM2HJxc

The Gauss Warthog clearly uses a Coilgun, which uses magnetic force to accelerate a projectile(similar to a railgun), but along an entirely different principle, as seen in that gif posted by the OP.

MAC guns are not lasers