A box cutter knife is a common tool used in grocery stores and other places. Such a knife generally consists of a simple and cheap holder, typically flat, approximately one inch wide and three to four inches long, which can hold a standard razor blade. The user can manually adjust how far the blade extends from the handle, so that the knife can be used to cut the tape sealing a package without damaging the contents of the package. When the razor blade becomes dull, it can be quickly switched for a new one.
A similar tool is known in British English as a "Stanley knife", after one of the first manufacturers to create this kind of implement.
Another style is one in which a handle, usually of plastic, contains a long, segmented blade which slides out from it. As the endmost edge becomes dull, it can be snapped off from the rest of the blade, exposing the next section which is sharp and ready for use. When all the individual segments are used, it is thrown away or a replacement blade is inserted.
Though such knives are not usually considered weapons, they were apparently used in the September 11, 2001 Terrorist Attack. See September 11, 2001 Terrorist Attack/Airport security for further discussion.
External link (Olfa cutters): http://www.olfacutters.thomasregister.com/olc/olfacutters/hd.htm
(Lots of pictures of the heavy duty-type cutters)