An ascender is a cammed mechanical device that allows a climbing type
rope to slide through the ascender one direction but not the other.
This makes ascenders great tools for either climbing up a fixed rope,
or pulling gear up a rope with out it being able to slide back down
the rope.
The ascender in the picture below is a
Jumar brand that was made in the 80's and still used today. This
Jumar's body is made of cast aluminum.
You can see the rope passes through a little trough, and is held
in place by a spring loaded cam. The ascender can be moved up the
rope, but when pressure is put on the ascender, the cam will not
allow the rope to travel up through the ascender.
Below are a couple of styles and brands
of ascenders for sale.