The laser assembly moves in only one plane, horizontally. After each horizontal scan, the printer moves the photoreceptor drum up a notch so the laser assembly can draw the next line. A small print-engine computer synchronizes all of this perfectly, even at dizzying speeds.
Some laser printers use a strip of light emitting diodes (LEDs) to write the page image, instead of a single laser. Each dot position has its own dedicated light, which means the printer has one set print resolution. These systems cost less to manufacture than true laser assemblies, but they produce inferior results. Typically, you'll only find them in less expensive printers.
Laser printers work the same basic way as photocopiers, with a few significant differences. The most obvious difference is the source of the image: A photocopier scans an image by reflecting a bright light off of it, while a laser printer receives the image in digital form.
Another major difference is how the electrostatic image is created. When a photocopier bounces light off a piece of paper, the light reflects back onto the photoreceptor from the white areas but is absorbed by the dark areas. In this process, the "background" is discharged, while the electrostatic image retains a positive charge. This method is called "write-white."
In most laser printers, the process is reversed: The laser discharges the lines of the electrostatic image and leaves the background positively charged. In a printer, this "write-black" system is easier to implement than a "write-white" system, and it generally produces better results.