Velcro is a trademarked 'hook and loop' fastener. It works by pressing both sides of the fastener together. One side is composed of small hooks, and the is composed of small loops. When pressed together the hooks tend to snare the loops and attachment occurs. Velcro is most commonly made of nylon and/or polyester. Depending on how well the hooks are ensnared in the loops a single square inch of velcro can hold up to 90lbs.