Flex is a measure of the club's ability to bend during the swing and is the single most important factor as it relates to overall distance and control. Golfers with faster swing speeds typically prefer stiffer shafts, which will improve accuracy, while those with slower swing speeds tend to hit better when their shaft has greater flex.
Torque relates to the way the shaft twists during the swing and is measured with a rating--the higher the rating, the greater the twist. Clubs with more torque will have a softer feel and give the ball a higher trajectory while ones with less torque, especially steel shafts, will give the ball a lower trajectory.
The kick-point refers to the place on the shaft where the bending actually occurs. A shaft with a low kick-point will usually give the ball a high trajectory and provide the feeling that the shaft is whipping the club's head through while a high kick-point has the opposite effect, giving the ball a low trajectory and make the shaft feel very stiff.
The weight refers to the actual uncut weight of the raw material before installation. Lighter shafts typically provide greater club head speed and distance. In general, graphite shafts tend to be lighter than steel shafts, but also have much greater flex.
A number of factors can adversely affect the alignment, from an imperfection in the material to the shaft not being perfectly round, which can affect the trajectory of the ball. A simple spine alignment should sort any issues out, ensuring that the spine of the shaft falls directly behind the golfer's target line.
In general, longer shafts provide greater distance and have more flex. However, what is most important is that each golfer find the proper length shaft for him or her. This can be achieved by ensuring that a shaft is cut based on the distance from the floor to the crease where the golfer's wrist and hand meet.