Hockey Stick Flex
Hockey stick shafts, much like golf club shafts, are highly flexible, and this flexibility is a key component in their performance. Flex, bend, stiffness, and whip are all terms used to describe the amount force required to bend a given stick shaft a certain amount.
With most composite and alumimum sticks, their flex characteristic is correlated numerically. This number, which ranges from 50 through 120, is printed on the stick and corresponds to the amount force (in pounds-force) that it takes to deflect or bend the shaft one inch. For example, 100 pounds-force (440 N) are is required to bend the shaft 1 inch (2.5 cm) and would be labelled "100 flex".
Stick flex is viewed as a very important characteristic by most players. Commonly, defencemen seek stiffer flex shafts, as their greater stiffness imparts more force on slap shots and improves stick-checking. Forwards will commonly seek more flexible shafts, as they require less force to bend and are thus better suited to create quick and accurate wrist shots, as well as improving passing and stickhandling.
Flex is also correlated to player strength; stronger players will often prefer stiffer flexes, as they have a sufficient amount of strength to fully bend (and thus maximize potential energy) using such shafts, whereas younger players and players with less strength will generally have more success using more flexible shafts which they are capable of bending to their optimal degree.
See also: http://en.wikipedia.org/wiki/Ice_hockey_stick
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)