Racquets were originally made of wood but most are now made of much stronger, lighter man-made materials such as aluminium or graphite.
The only restrictions on the racquet are its size.
It must be no longer than 29 inches (73.66cms) including the handle, or 12.5 inches (31.75cms) in width.
The hitting surface must be a flat surface of criss-crossed strings.
It should be no more than 15.5 inches long (39.37cms) or 11.5 inches wide (29.21cms).
Most racquets are strung with synthetic materials such as nylon or even titanium.
But the top players prefer to use 'gut' strings. Gut is made from the dried stomach lining of animals such as a cow or cat.
Big racquets have bigger 'sweet spots' but are harder to control.
Roddick tests out his sweet spot
A 'sweet spot' is the area in the middle of the strings which gives the cleanest hit. A mid-size racquet has a hitting area of about 85-95 inches.
Beginners are sometimes encouraged to buy oversized racquets - up to 105 square inches of strings.
Mid-size and mid-plus are usually preferred by intermediate and advanced players. A third option, mid-size plus (95-105 square inches), may be the best option as it will still be suitable as you improve.
Most players should go for a reasonably slim frame to ensure reasonable flexibility (some pros like big thick frames which are very stiff).