In a simple language ball tampering means altering the condition of the ball. The prime motive of this act is to interfere with the aerodynamicsof the ball.
UnderLaw 41, subsection 3 of theLaws of Cricket, the ball may be polished without the use of an artificial substance, may be dried with a towel if it is wet, and have mud removed from it under supervision.
All other actions which alter the condition of the ball are illegal. These are usually taken to include rubbing the ball on the ground, scuffing with a fingernail or other sharp object, or tampering with theseam of the ball.
The purpose of altering the state of the ball is to achieve more favorable bowling conditions for bowlers. However, rubbing spit and sweat on a ball while bowling to the batsman is legal. The moisture gained from spit or sweat when combined with polishing, smoothens out one half of the ball which in turn allows air to pass over one side of the ball more quickly than over the other.
When bowled correctly, a bowler can get the ball to move from one side to the other through the air. Also, it is common for bowlers to rub the ball against their clothing to dry or polish it, as seen in most cricket matches.

