audit is a checking done on roles n responsibilities of a 
job with prior intimation.This audit is of 2 types.
1)Internal audit
2)External audit

internal audit:-audit done by QA/QC within the company is 
known as internal audit.

External audit:-audit done by third party is known as 
external audit.

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An independent evaluation of sotware products or processes 
to acertain compliance to 
standards,guidelines,specifications, &/or procedures based 
on objective criteria,including document that specify:

1--The form or content of the product to be produced.

2--The process by which the products shall be produced.

3--How compliance to standards or guidlines shall be 
mesured.

This can be internal or external as disscuss above .