Internal audit is get done by directories or management of the company where as externals audit is get done by shareholders or tax authority or bankers etc.

In general, internal audit is not mandatory or legally required  for organizations. Instead, their installment is left up to individual organizations’ discretion. Recent legislation, however, has made internal auditing mandatory in some cases. Internal audit is done by internal staff.

An external audit is legally required for many companies, External audit is conducted by persons independent of organisation.