Commonly known as a Jumar - very simple and safe way of ascending ropes but a tad heavy for the alps. One is often required for ascending fixed ropes on guided himalyan peaks - in which case you should choose a right-hand one if you are right-handed and a left-hand one if you are left-handed. Can be used on ropes from 8mm - 13mm. Very slightly revamped version of the original.
196g.
For more information on the uses of an Ascender see the Petzl PDF.