Definition : A consonant sound sound made by producing a plosive and then a fricative in the same place in the mouth.
Examples : The only English affricates are /ʧ/ and /ʤ/. /ʧ/ = unvoiced palato-alveolar affricate /ʤ/ = voiced palato-alveolar affricate - ie made with the tongue touching the midway point between the alveolar ridge and hard palate.
To hear recordings of affricates used in other languages see here
More reading from An ELT Notebook : English Consonant Sounds
