The laser is not often used as a weapon, as it does not do much damage - but it's a useful tool to move around the map, because it has a significant push. The 'explosion' from its projectile will push you and other players around a lot. The trick to using it to move around is to use this push to gain speed or to do huge jumps. For an easy start, take the laser, look down at the floor, and press fire. It will push you high into the air. If you press jump and fire at the same time, you will get even higher - but it depends on the timing, and, for more advanced jumps, also on the angle you fire at the floor, as well as how fast and in which direction you move. For example if you just run forwards, look down and press fire it will catapult you forward with great speed. Play around with it, and watch others, and you will learn a very useful skill in Nexuiz. The secondary fire mode of the laser switches back to the last used weapon. The laser does not use up any ammo.

The Laser is your other starting weapon. If you can't remember which one it is, it's the one that isn't a shotgun.
At first glance, it appears to be a completely useless weapon. Although it has unlimited ammunition, the projectiles do low damage, the rate of fire isn't anything spectacular, and it just appears to bounce players around quite a bit. However, perhaps somewhat paradoxically, this is its greatest strength.
Its main intended use is as a movement tool, more than as a weapon. When used in a fashion similar to rocket-jumping, it can be used to “laser jump” to impressive heights or distances, and to accelerate to impressive speeds, which is particularly important in CTF.
Also, since it tends to bounce other players around, a much decried but effective use is to use it on space maps to bounce other players into the void.