Paracord (also parachute cord, P-cord or 550 cord) is a lightweight nylon kernmantle rope. It has a core of 7 white inner strands wrapped in braided colored outer layer.
Because it's made of nylon, it needs to have its ends melted in order to keep from unravelling.
Step 1: History
It was originally used in the suspension lines of US parachutes during World War II. Once in the field, paratroopers found this cord useful for many other tasks. It is now used as a general purpose utility cord by both military personnel and civilians. This versatile cord was even used by astronauts during STS-82, the 82nd Space Shuttle mission, to repair the Hubble Space Telescope.
The same properties which soldiers appreciate in paracord are also useful in civilian applications. After World War II parachute cord became available to civilians, first as military surplus and then as a common retail product from various surplus stores and websites. While some commercially available paracord is made to specification, even when labeled as such a given product may not correspond exactly to a specific military type and can be of differing construction, quality, color, or strength. Particularly poor quality examples may have significantly fewer strands in the sheath or core, cores constructed of bulk fiber rather than individual yarns, or include materials other than nylon.