Bug can be defined as the abnormal behavior of the software.
No software exists without a bug. The elimination of bugs
from the software depends upon the efficiency of testing
done on the software. A bug is a specific concern about the
quality of the Application under Test (AUT).
Bug Life Cycle:

In software development process, the bug has a life cycle.
The bug should go through the life cycle to be closed. A
specific life cycle ensures that the process is
standardized. The bug attains different states in the life
cycle. The life cycle of the bug can be shown
diagrammatically as follows:

The different states of a bug can be summarized as follows:

1. New
2. Open
3. Assign
4. Test
5. Verified
6. Deferred
7. Reopened
8. Duplicate
9. Rejected and
10. Closed
Description of Various Stages:

1. New: When the bug is posted for the first time, its state
will be ?NEW?. This means that the bug is not yet approved.

2. Open: After a tester has posted a bug, the lead of the
tester approves that the bug is genuine and he changes the
state as ?OPEN?.

3. Assign: Once the lead changes the state as ?OPEN?, he
assigns the bug to corresponding developer or developer
team. The state of the bug now is changed to ?ASSIGN?.

4. Test: Once the developer fixes the bug, he has to assign
the bug to the testing team for next round of testing.
Before he releases the software with bug fixed, he changes
the state of bug to ?TEST?. It specifies that the bug has
been fixed and is released to testing team.

5. Deferred: The bug, changed to deferred state means the
bug is expected to be fixed in next releases. The reasons
for changing the bug to this state have many factors. Some
of them are priority of the bug may be low, lack of time for
the release or the bug may not have major effect on the
software.

6. Rejected: If the developer feels that the bug is not
genuine, he rejects the bug. Then the state of the bug is
changed to ?REJECTED?.

7. Duplicate: If the bug is repeated twice or the two bugs
mention the same concept of the bug, then one bug status is
changed to ?DUPLICATE?.

8. Verified: Once the bug is fixed and the status is changed
to ?TEST?, the tester tests the bug. If the bug is not
present in the software, he approves that the bug is fixed
and changes the status to ?VERIFIED?.

9. Reopened: If the bug still exists even after the bug is
fixed by the developer, the tester changes the status to
?REOPENED?. The bug traverses the life cycle once again.

10. Closed: Once the bug is fixed, it is tested by the
tester. If the tester feels that the bug no longer exists in
the software, he changes the status of the bug to ?CLOSED?.
This state means that the bug is fixed, tested and approved.

While defect prevention is much more effective and efficient
in reducing the number of defects, most organization
conducts defect discovery and removal. Discovering and
removing defects is an expensive and inefficient process. It
is much more efficient for an organization to conduct
activities that prevent defects.
Guidelines on deciding the Severity of Bug:

Indicate the impact each defect has on testing efforts or
users and administrators of the application under test. This
information is used by developers and management as the
basis for assigning priority of work on defects.

A sample guideline for assignment of Priority Levels during
the product test phase includes:

   1. Critical / Show Stopper ? An item that prevents
further testing of the product or function under test can be
classified as Critical Bug. No workaround is possible for
such bugs. Examples of this include a missing menu option or
security permission required to access a function under test.
      .
   2. Major / High ? A defect that does not function as
expected/designed or cause other functionality to fail to
meet requirements can be classified as Major Bug. The
workaround can be provided for such bugs. Examples of this
include inaccurate calculations; the wrong field being
updated, etc.
      .
   3. Average / Medium ? The defects which do not conform to
standards and conventions can be classified as Medium Bugs.
Easy workarounds exists to achieve functionality objectives.
Examples include matching visual and text links which lead
to different end points.
      .
   4. Minor / Low ? Cosmetic defects which does not affect
the functionality of the system can be classified as Minor Bugs.

In these bug life cycle first stage is when the tester 
fined the bug regisrted in the bug tracking tool
open=then bug will be opened by the developer.
fixed= then bug will be fixed by the developer.
closed== then bug will be closed by the tester after 
verification. 
reopen== then if the same bug is reproducing the bug will 
be reopend by the tester.