Head Size:

The vast majority of racquets are in the  97 to 105-square-inch range. Decades ago, it was common to see racquets with head sizes of 90 square inches (or less!), but they have skewed larger over time to provide the best combination of forgiveness, maneuverability, and stability. 
Provided your swing speed stays constant, a larger racquet head will produce more power than a smaller racquet head. For example: A 95 in² head will produce much less pop than a 110, while a 97 and 98 would be difficult to differentiate.
Midsize: 85-95 in² (550-615 square centimeters) 
Mid Plus: 96-105 in² (621-680 square centimeters) 
Oversize: 106-118 in² (686-761 square centimeters) 
Super Oversize: 119 in² and above (767+ square centimeters)

 

Balance

A  head-light racquet means more of the weight is housed in the handle. This balance is preferred by players who spend a significant amount of time at the net. Conversely, the  head-heavy balance places more weight in the racquet head. These frames are more cumbersome at the net, but can provide more stability and power from the back of the court.   Super-light racquets are often head heavy to help produce power, whereas middle-weight or heavy racquets are more evenly balanced or head-light for maneuverability.
Flex / Stiffness

Flex and/or stiffness generally affect a racquet’s   feel, which often boils down to personal preference. A more flexible racquet means the ball is staying on the strings a fraction of a second longer, providing additional  shock absorption, plus increased control and comfort. Usually racquets with  thinner beams (20-22mm) will be more flexible frames, while racquets with  thicker beams will have a firmer feel.

Swingweight  (measured in kg/cm²) 

Players like Andy Murray and Novak Djokovic use very  high  swing weights – well above 350! Most racquets fit in the  300-330 range. The higher the number, the more cumbersome it is to get the racquet moving. The reward for a high swing weight can be added power, but only if you can accelerate quickly from the start. Racquets with  lower swing weights are more maneuverable, but can also get pushed around by the ball.

For more racquet-related definitions and details, head to our  Key Tennis Terms page.