What are the difference between internal auditor and external auditor?

An internal auditor is normally an employee of the organisation who is trained in auditing and depending on the company requirement, would carry out annual or bi-annual audits.

External audits are carried out by a company not affiliated in any way to the organisation being audited. This can be a regular routine set up whereby the external auditor comes into the organisation every 3 years or so or it could be the external auditors are called in to do some investigation if there is suspicion somewhere.