Technically it is known as a “hook and pile fastener” but Velcro is the most popular brand.
So how does Velcro work?
Velcro works by having two strips of material, one with lots of tiny hooks, and one with lots of thinner loops that the hooks can cling to when the two elements are pressed together. This mechanism of sticking was inspired by the way tiny hooks on the seeds of burdock, known as burrs, attach to animal fur and human clothing.
The hooks and loops in Velcro are commonly made from nylon and polyester. The more hooks and loops that are attached per unit area, the stronger the bond.
Here is a video demonstration of how Velcro works, shot with a super macro lens — which is also oddly satisfying.