Lasers consist of a laser emitting device, and the laser beam. The laser emitting device is square, and has three tubes sticking out of the back into the wall, probably to get power. The yellow hemispherical circle is what actually emits the laser beam.
The laser beam, commonly known as laser, is the main weapon emitted from the laser. If Zapo touches one, it will instantly die, and will be respawned from its last touched restart pad. Zapo can be protected from them if it folds them away with a fault line. They are the only thing that can destroy liquid holding blocks.