A printer drum is a rolling pin inside of a laser printer. To create image patterns, a laser beam shines on the drum in the shape of the print out, to place what are called "electrostatic images." The drum is then coated with toner, the powdered substance that creates printed images on the paper (for both black and coloured prints), on those previously invisible electrostatic patterns. The paper rolls through the drum, where the toner is applied, and then finally goes through what is called a fuser (hot to melt the powder) to create your printout.
As you can see, the drum is extremely important to the proper operation of a laser printer. When it goes bad, you will have serious issues with print quality and you may experience frequent paper jams.
When a laser printer is used excessively, the printer drum can go bad more quickly than it is supposed to. Many printer manufacturers will give you a recommended usage for the printer. For instance, a good laser printer will have a maximum monthly duty cycle of about 20,000 sheets per month. Anything beyond that and you are working the drum way too hard and too frequently. You have to give the printer a rest during the day--allow it to cool down and recharge itself.
When you use the wrong printer paper in your laser unit, it could cause problems with the drum. It is a very bad idea to use inkjet paper in a laser printer. The drum might not be able to transfer all of the toner onto the paper correctly, and then you will have issues with toner being left behind on the drum. Also, inkjet paper could melt when it goes through the heated fuser, causing additional problems.
Time--Simple Wear and Tear
Over time, even if you use your printer sparsely, your drum is going to wear out and go bad. The drum has a definite expiration date. Some printer manufacturers will tell you approximately how many pages the drum will print over the course of its lifetime before you will need to buy a new one (or a whole new printer).
When the Drum Goes Bad
The drum is pretty expensive to replace, so when problems arise, you will have to make a decision as to whether you want to get a new one, or just purchase a new laser printer. It is possible to clean a drum in an attempt to repair it yourself, but it is very fragile and you could just end up damaging it beyond repair. Additionally, toner powder is a toxic substance until it is transferred to paper, so it's not a good idea to fiddle around with the drum unless you really know what you are doing.