By allowing ordinary people (that's me) to produce printed documents that look almost like they were professionally published, and to do it all in their own home or office, laser printers represent a true technological revolution. The first relatively affordable laser printers were the Apple LaserWriter and the Hewlett-Packard LaserJet. Prices have dropped like a rock and quality is way up since those machines came out, but the basic technology remains the same.
Laser printers work by laying down an array of tiny, evenly spaced dots of "ink" on the paper. The dots are so small and they blend together so seamlessly that text looks very nearly as clean as what you get from a traditional typesetting machine. Yes, there are little jagged edges along curved lines, but they're hard to see. Graphics sometimes don't turn out quite as sharp, but they still are clear enough for a professional look in many situations.
The quality you get from your laser printer depends mostly on the resolution, the fineness of the dots it uses to print the images. Resolution is measured in dots per inch (dpi), which is how many dots it can print along a line, either vertically or horizontally. Most laser printers have had 300 dpi resolutions, but 600 dpi is just now becoming the new standard. If you're shopping for a new printer and are willing to spend around $2,000,make sure you buy a 600 dpi machine-it's really tough to see any jagged edges.
Although the output of a laser printer is sometimes described as "near typeset quality," don't mistake the mechanical resolution with the aesthetic quality of good typography. No matter how high the resolution, if you still use typewriter conventions (such as two spaces after periods, straight quote marks [" instead of" and "] two hyphens instead of a dash, underlines, and other typographic faux pas), the text will never really look "typeset quality," even if you use 2540 dpi. If you are still using typewriter conventions and trying to produce high-quality output, you should check out the books The Mac is not a typewriter or The PC is not a typewriter.
Behind the scenes: a laser printer relies on a process something like that used to generate images on a television. In a TV set, a beam of electrons rapidly scans across the video tube, building an image out of tiny dots of light. In a laser printer, a tightly focused laser beam does the scanning. Although the laser source itself is stationary, a rapidly spinning mirror directs the laser beam so that it quickly traces a narrow, precisely horizontal line from one side of a special light-sensitive drum to the other. Initially, this drum carries a uniform static charge. As the laser beam moves across the drum, however, it flashes on and off, and wherever it touches the drum it reverses the static charge of a minute dot.
When the beam reaches the far edge of the drum, the drum rotates precisely. The laser beam then scans back across the drum, continuing to flash on and off. This cycle repeats itself until the laser has etched the drum with a pattern of charged dots corresponding to the entire page to be printed.
Even before the laser beam has finished its work, the drum's slow rotation brings the portions already etched into contact with the toner, a black plastic powder that serves as the printer's ink. The toner itself is charged so that it sticks to the dots etched by the laser. Eventually, all the etched areas of the entire drum receive a coating of toner. Meanwhile, a sheet of paper has been drawn into the printer by a system of rollers. In the process, these rollers have given the paper a static charge of its own, but one stronger than the charge on the drum. As the drum presses against the paper, the paper's charge attracts the toner away from the drum and onto the paper. More rollers move the paper into the fusing system, where heat and pressure affix the toner permanently to the paper. That's why the paper feels a little warm when it emerges a few moments later from the printer. As this is going on, a thin corona wire restores the drum's original homogeneous charge, readying it for the next page. Whew.