Strung Weight

This is the weight of the racquet with strings installed, also referred to as playing weight. When all other specifications are the same a heavier racquet will be more powerful provided the player's swing speed can maintain the same acceleration. However, if you play an aggressive, spin heavy baseline style of play, a heavier racquet may not increase your power due to the increased difficulty the weight would create. Your style of play and fitness will help determine what weight range you can play with. Nadal and Federer play with very different swing styles and therefore play with very different weight racquets. Despite Nadal being physically larger and assumedly stronger, he actually uses the lighter racquet.

Tips

A heavier racquet will generate more power provided you have the same swing speed.
Heavier racquets tend to be more stable at the point of contact (this is a generalisation, as balance and swingweight affect stability).
Lighter racquets will be more manoeuvrable due to decreased force required for movement (this is a generalisation, as balance and swingweight affect manoeuvrability).
Reducing weight is basically impossible, but adding weight is quite simple.
Strength, swing speed and swing style are important to consider when looking at the weight of a racquet.