Most of the basic information newer posters need is already available in the learning center.
http://www.tennis-warehouse.com/LC/SelectingRacquet/SelectingRacquet.html
"Frame Stiffness
The amount a frame deflects during ball contact directly affects its power potential. A stiffer racquet bends less, thus depleting less energy from the ball. A flexible racquet bends more, resulting in more energy loss. A common myth among players is that a flexible racquet, that bends back more, returns more power to the ball due to a catapult-effect. The ball remains on the strings for 3-5 milliseconds, much shorter than it takes a frame to recover. Consequently, a racquet frame doesn’t “return” energy to the ball, it absorbs energy - either more or less, depending on stiffness. Stiffer racquets don’t deflect as much on impact, resulting in less power drain than a flexible racquet.
Frame stiffness doesn’t only affect power though. Control and comfort are also at stake. Generally speaking, a racquet that offers more power provides less control. However, this is largely dependent on player type and ability. An advanced player may prefer a flexible racquet because he or she has a long, fast swing and provides plenty of power. A stiff racquet might be too powerful for this player, resulting in too many balls landing long. A beginning or intermediate player though, may find a stiffer racquet that doesn’t deflect as much on impact, provides better control. This may also apply to a more advanced player with short, compact strokes. To a certain point, stiffer racquets are generally less comfortable than more flexible racquets. A very stiff frame will transmit more impact shock to the wrist, elbow and shoulder than a medium stiff frame. Comfort is difficult to measure - each player has a different perception of what feels comfortable. However, players with arm and/or shoulder problems will generally benefit more from a flexible to medium-stiff frame and should likely avoid stiff or very stiff frames. A lesser known effect of frame stiffness is the amount of spin that can be generated. Generally speaking, stiffer racquets provide less spin than flexible racquets because the ball leaves the stringbed more quickly. "
