A stop sound is pronounced with a sudden release of breath. In other words, it is produced by stopping air at some point and suddenly releasing it
Affricative sound is a composite speech sound consisting of a stop and a fricative articulated at the same point (as `ch' in `chair' and `j' in `joy').
The following are the explanations from Answers.com
Any speech sound made by impounding the airstream for a moment until considerable pressure has been developed and then suddenly releasing it (e.g., b, d, and g). One of two types of stop consonants, the other being affricative (e.g., ch).
A fricative speech sound initiated by a plosive.