BUG : It occurs on development side (when Compile time or Runtime )ERROR : The Test Engineer check the application and if he find the conflict, it is known as Error.DEFECT : It founds on UAT side..

Bug is the problem found at the developers site.

When the developer doesnot correct/found it and reached to the tester and tester reports it it becomes error.

And if it has reached to the customer and customer find it , then it becomes defect.

Error : It is the Deviation from actual and the expected value.

Bug : It is found in the development environment before the product is shipped to the respective customer.

Defect : It is found in the product itself after it is shipped to the respective customer.