A shopper's guide
Badminton racket heads come in two shapes: isometric (square) and oval. Oval is the more traditional shape (this type of head is often bottom-heavy), and provides a small but concentrated sweet spot. An isometric head, on the other hand, has a wider, squared top half and offers a larger sweet spot, giving an off-center hit a better response.
The stiffness of a racket has the same effect as its string tension; i.e., a stiffer racket will perform like a racket with high string tension, and a flexible racket will perform like a racket with low string tension. High tension favors hard hitting and steals power from a light hitter, while low tension favors light hitting, providing a well-timed trampoline effect.
As for weight, the lighter the racket, the faster a player can swing it without greater force. A light racket is more maneuverable than a heavy one, but a light racket is also less stable, and greater force is required to keep its path consistent. A heavy racket provides greater momentum and is better at transferring speed and power to the shuttlecock.
Finally, balance is an important factor to consider when buying a racket. Head balance is becoming more and more popular these days. Head-balanced rackets have greater mass near the head, are more stable, and offer higher angular momentum when swung, whereas even-balanced rackets are more maneuverable.
In the end, however, it's a player's experience and skill that will determine which racket he or she should buy, and the only real way to decide on a racket is by trying out as many as you can and seeing what fits.