What is a bug and bug life cycle? What are guidelines for deciding severity of bugs?

A bug is defined as a defect or some abnormal behavior of software. Testing plays an important part in the removal of bug. Bug has to travel the whole bug life cycle until it is closed. The cycle includes following stages:
- New
When the bug is posted for first time and not yet approved.
- Open
When tester approves that bug is genuine.
- Assign
Bug is assigned to the developer.
- Test
After fixing the bug, it is assigned to testing team to re-test it.
- Deferred
When the bug is changed to deferred state, the bug is expected to be fixed in next releases.
- Rejected
If the developer feels that the bug is not genuine, he can reject the bug.
- Duplicate
If bug is repeated twice or two bugs gives the same concept, then one bug is labeled duplicate.
- Verified
Once the bug is fixed, it is verified that no bug is present and status is changed to verified.
- Reopened
In this stage, the bug traverses the bug cycle once again because the bug still exists.
- Closed
If the bug is fixed and does not exist, the tester changes the status to closed.

SEVERITY AND PRIORITY OF THE BUG HAS TO FOLLOW GUIDELINES:
- Critical bug prevents further testing of the product under test. No work around is possible for such bugs.
- Major bug is in which defect does not function as expected or cause other functionality to fail.
- Medium or average bug in which defects do not conform to standards and conventions.
- Minor or low bugs do not affect the functionality of the system.

To write bug description, follow these guidelines:
- Be specific.
- Use present tense.
- No unnecessary words.
- No exclamation points.
- Do not use all CAPS.
- Mention steps.