What is parachute cord?
Parachute cord, also known as paracord, is a variety of what is known as kernmantle rope, which is comprised of the kern (core rope(s)) and is encased by a mantle ( sometimes referred to as a jacket). Kernmantle ropes gains their strength from the kern and durability from their mantle.
What is parachute cord made of?
Paracord, unlike some kernmantle ropes, has a core and and jacket both made from the same material: nylon. This results in a durable rope with a smooth touch and mild stretch. Since its original field use by paratroopers in WW2 paracord has been adopted by most branches of the US Armed Forces and even by astronauts while repairing the Hubble Space Telescope.
What is paracord used for?
In the field paracord has advantages over other types of rope and cordage in that it can be tactically deconstructed for a wide variety of survival applications. By "gutting" or removing the kern from the mantle the core can be separated into smaller, finer stands. These strands are useful as fishing line or sewing threads. The gutted mantle remains useful as a cord with a smaller girth and stretch, making it perfect for tie downs and bootlaces.
Civilian use has become commonplace with hikers and other outdoors enthusiasts. Paracord's lightweight and compact design make it easy to carry large amounts. It is often fashioned into bracelets or other decorative, yet functional, items that can be quickly deconstructed in emergency situations.
Sources: Parachute Cord: Wikipedia