Laser printers are flexible business tools because in addition to regular paper they can print on a number of different specialty papers, labels, card stock and transparencies. The key thing is that laser printers have a component called a fuser, which uses high heat to melt the toner onto the paper. Media that's designed for laser printers can withstand the heat of the fuser.
Most laser printers will provide excellent performance on the standard paper that most businesses use for all of their printers. However, special laser printer paper typically has a slightly lower moisture content than regular office paper. This can help to reduce paper curl and wrinkling, especially if your printer is located in an area with high humidity levels.
Many specialty papers are incompatible with laser printers. Inkjet photo paper, iron-on transfer papers and the like all use special chemical coatings that can be melted by the heat in the fuser, damaging the printout and, potentially, your property. Some manufacturers make specialty papers, though, that are designed to withstand the heat of your laser printer. If the paper is marked as being designed for "laser" or "laser/inkjet" printers, you can safely use it.
Laser printers can print on some types of card stock. To find out what weight of card stock your printer can handle, look at your user's guide. Some printers can't handle heavy card stocks because overly thick paper won't fit through the bends and twists of their paper paths. In addition, some printers require you to put card stock in a different paper tray or have them output to a different tray, allowing for a straighter paper path.
Envelopes and labels pose special challenges for laser printers because of their adhesives. Products designed for laser printers use special glues for their envelope flaps or label backs that can withstand the fuser. Sheets of labels designed for laser printers also typically have paper covering every bit of their wax backing paper -- preventing the wax from melting onto the fuser.