Actually:
Bug:       Deviation from requirement is a bug.
Deffect:   Defect is the one which wil cause the software
           failure.
so Bug is Cause for defect
   Defect is effect of the bug.

One more thing in real time people use  these both terms as
Synonymical words. Majority of people say both are same.

still any one of have doubts regarding this concept feel 
free to call or mail me

Byee  

rajendra_penumalli@yahoo.com
9885162742
hyderabad