"(a) Any fielder may

(i) polish the ball provided that no artificial substance is used and that such polishing wastes no time (England's John Lever comes to mind).

(ii) remove mud from the ball under the supervision of the umpire (Pat Symcox who was commentating at the time, felt that Sachin Tendulkar was merely cleaning mud off the seam. But since he was not doing it under the supervision of the umpire, he may have contravened this provision).

(iii) dry a wet ball on a towel.

(b) It is unfair for anyone to rub the ball on the ground for any reason (Bedi, Chandrashekar and Prasanna did it quite openly and legally when we used to open the bowling attack with the likes of Gavaskar and Solkar), interfere with any of the seams or the surface of the ball, use any implement, or take any other action whatsoever which is likely to alter the condition of the ball, except as permitted in (a) above."

This article from The Daily Mirror, posted on a newsgroup, explains the methods in which a ball could be tampered with.

A reason to tamper with the ball is to enable one to generate prodigious movement and swing. This Web page explains the aerodyanamics, science and art of swing bowling. Allegations of ball tampering have been made against a slew of international players. Pakistan's Waqar Younis was perhaps the first bowler to be suspended and fined for ball tampering.
Tendulkar was accused in early 2000 by Abdur Razzaq in Australia while playing a league match against Pakistan, but was later cleared. Two other ball tampering cases that made headlines were Michael Artherton, and Hansie Cronje.

For more information on ball tampering you can also try Rediffsearch.

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