The generic name for Velcro.
Velcro is a trademark, and Velcro Industries is aggressive about protecting it. They have to be - like Band-Aid, Elastoplast, spackle and Frigidaire, velcro is at risk of losing its brand status. So if it doesn't say VELCRO®, don't call it Velcro.
The generic stuff works just as well, though.
How it works
Hook and loop fastenings are based on the way that a cocklebur sticks to a wool sock. (see Velcro for the story)
There are two pieces, a hook piece and a loop piece. The hook piece has rows of tiny plastic hooks, based on those on a cocklebur. The loop side looks like tiny loop-pile carpet, made of fine thread.
When you press the two pieces together, each hook catches on a bit of thread in the loop side. The plastic of the hooks is thin enough that they can straighten up. As they straighten, they release the loops with a slight tearing sound.