Bug: it is inconsistancy netween Functional Requirement and 
actual implementation. If something is already documented 
to be implemented and implementation is not consistant as 
per requirement then its a BUG.

Defect: If something is happening with the functionality 
that is not mentioned in any document to be followed during 
implementation or testing then its a defect.
Like if we double click on any Folder existing on the root 
node and get an Internal Error or System Crash message then 
its a Defect.