It is usually a thin rod bent into a looped shape that takes advantage of the flexibility and strength of the materials of its construction (usually some sort of metal, but sometimes plastic) to compress and therefore hold together two or more pieces of paper.
A Norwegian, Johan Vaaler, patented what is still the commonest shape in 1901. The paperclip became a Norwegian national symbol in World War II as people wore them in their lapels when the Nazi occupiers forbade buttons showing the exiled King Haakon VII of Norway. A giant paperclip was erected in Oslo in honour of Vaaler.
Despite hundreds of variations, the original design is still the most popular. Its qualities of easy use, gripping without tearing, and storing without tangling have been difficult to improve on.
Recent innovations include multi-colored plastic-coated paperclips and spring-fastened binder clips.
A paperclip is also a useful accessory in computing: the metal wire can be unfolded with a little force. Several devices call for a very thin rod to push a recessed button which the user might only rarely need. This is seen on most CD-ROM drives as an "emergency eject" should the power fail; also on early disk drives (including the early Macintosh). Some Palm PDAs advise the use of a paperclip to reset the device.
Microsoft infamously introduced a cartoon character shaped like a paperclip in their Office 97 software suite, nicknamed "Clippy". It was intended to deliver useful information to users as an active help procedure, but it was largely perceived as an irritation by users, and became the subject of many parodies. In newer versions of the software, it is switched off by default. Clippy had been preceded by "Bob", whose name is still attached to the Clippy software.