Laser printers are found mostly in businesses. The laser printing process is fairly complex. A laser then removes charges from parts of the photoconductor leaving only the charged image of the document. This pattern of toner is then transferred to paper with high heat and pressure. I told you it was complex.
Laser printers are very fast and can achieve extremely high print quality.
If you do a lot of text printing, a laser printer can be a cost effective purchase. Higher numbers equal higher quality. Today's printers average 600dpi or higher. However, the human eye has trouble seeing the difference once you get above 150dpi. In other words, PPI determines the speed of the printer.