An internal audit is done within the company or by some one hired by the company and an external audit is by someone who is not hired by the company example a board of health or other agency

 Internal Audit is the audit which organization's own department which name can be Audit department do to check the function of finance department and as well other departments. Aduit department checks the Accounts of the organization and estimates the year budget.
External Audit is the audit the organization calls other Audit organizations to check their records. 

 External audit is the one which is taken by the head of company branch while the internal is the audit taken from the same branch this link will help you out in getting it.