The FIFA regulations on the ball are found in FIFA 2014/2015 Laws of the Game (PDF), Law 2:
LAW 2 - THE BALL
Qualities and measurements
The ball is:spherical made of leather or other suitable material of a circumference of not more than 70 cm (28 ins) and not less than 68 cm (27 ins) not more than 450 g (16 oz) and not less than 410 g (14 oz) in weight at the start of the match of a pressure equal to 0.6 – 1.1 atmosphere (600 – 1,100 g/cm2 ) at sea level (8.5 lbs/sq in – 15.6 lbs/sq in)
These are the general regulations for any ball, whether or not it is used in the World Cup.
However, as is the case with every World Cup, they have developed a new ball to be used in this year's 2014 FIFA World Cup in Brazil. It is the Adidas Brazuca:
The specifications for the Adidas Brazuca are:Six polyurethane panels 437 grams 69 cm circumference 0.2% water absorption 141 cm rebound 7% loss of pressure ...