These are shredders best suited for consistently modest volumes of paper in the standard 8 1/2 x 11" format. Usually they can handle 5 or 6 sheets at a time and have an output capacity of around 6 gallons. They run on 110-volt power and are about the size of a waste basket: typically they sit off to one side of the desk. They can be used for all correspondence and advertising material that fits. A good auxiliary tool is a pair of scissors or a paper cutter! Note that they are not meant for continuous operation.
Both strip and crosscut styles are available. Some have sensors to stop the machine if it's in danger of overheating. Some have nice "bells and whistles" like castors on the output hopper so you don't have to carry it - six gallons of paper can be quite a load; an angled throat (instead of a vertical one) so you don't have to pull it out to load if it's under your desk; windows in the output box so you can see if it's getting full; a reverser switch to help clear jams; and a safety lock for safety purposes.
The Fellowes "Powershred DS-1 Crosscut" shredder is one that is doubly worth a look if there are curious kids around: it has an interlock that will stop the machine if the paper feed throat is touched.
The GBC "CC195 Personal Crosscut shredder" is an example of models worth looking at if space is at a premium: it is very narrow and wider than most shredders; it's meant to be parallel to the side of the desk, not the back.
Recently, a few very low-end manual shredders have entered the market appealing to the desire to avoid identity theft. For about $20 you can purchase a "3-in-1 Shredder," which is basically a little box with a slot capable of accepting 8 1/2-inch-wide paper. It's operating by turning a crank, which moves steel cutting blades that shred the paper. It also has slots to shred CDs and credit cards.
There is also a product called the Ziszor, a portable handheld shredder that is battery operated for shredding at home or on the road. It has 28 steel cutting blades, a shredding capacity of five sheets at a time and uses specially designed catch bags.
There are all sorts of choices worth looking into. Generally, as in most consumer products, it's best not to buy the cheapest possible rig as it will cause more trouble than it's worth and will conk out quickly. All else being equal, get the one with the best warranty - local service preferred, of course.