Error : A mistake in coding is called Error.
Defect or Issue : When tester find that mistake during test 
execution, test engineers called defect or issue.
Bug : When developers accepted that defect to solve , that 
is called bug.

a mistake in coding is called error
tester find mistake in testing time is called defect
whenever developer fix that defect is called bug.

An error in coding identified through sample executions 
(otherwise at the testing phase) is called a defect.

This identified defect which does not throw out the ER=AR
is a bug present in the software.

However its easy for one to consider bug and defect to mean 
one and the same thing as there is a very fine line between 
the two.

Bug and Defect are same.
Informal name given to Defect is called as Bug.

Finding the errors is called Bug/issues some people says  
When tester find that mistake during test execution is 
called 
Client side people identify the errors is called defect

A mistake in coding is called as an error.

A mistake found by the tester in called as bug.

A mistake found by the end-user is called as defect.

*A mistake found in testing environment is called a Defect.

*If developer agree to solve the bug  is called as bug.

Defect and Bug are same but usage wise its differ..

I think Ashok Kumar's solution is more apt one for this 
query.

HumanBeing is made for Mistake.

When a developer makes a mistake (Syntactic or Semantic) in 
an application that is called Error in coding.

When a tester while testing finds that error hile 
execution,it is called Defect (or Bug).

At the system level testing of the whole application if u 
find a defect in the software,it is called Fault.

If End user finds that the software is not as per the 
requirement,it is called Failure)

Error in the coding are called as bug

Defect means finding the bug  and rectified by the 
developer

when a developer identified an error during coding is called
Bug.

When the tester test the application and find the issues it
is called defect.

(Whenever a build is deployed and the tester finds the
issues is called defect).

Bug and Defect both are same.

Tester uses the term Defect when talking about 
functionality, where as developer uses the term bug talking 
about the Code.

Defect is non conformance to the requirements and the test case.
Bug is a undesired functionality crept in. This could be
because of a scenario not thought of during requirements
gathering.
The best example of a bug is the Y2K. most of the systems
worked up to 1999 and couldn't round of to year 2000.
The floor and ceil is another example - 14.3 when rounded
off should round off to 14 and not 15 as per majority of the
requirement.