Internal auditors are company employees, while external auditors work for an outside audit firm.
Internal auditors are hired by the company, while external auditors are appointed by a shareholder vote.
Internal auditors do not have to be CPAs, while a CPA must direct the activities of the external auditors.
Internal auditors are responsible to management, while external auditors are responsible to the shareholders.
Internal auditors can issue their findings in any type of report format, while external auditors must use specific formats for their audit opinions and management letters.
Internal auditors can be used to provide advice and other consulting assistance to employees, while external auditors are constrained from supporting an audit client too closely.
Internal auditors will examine issues related to company business practices and risks, while external auditors examine the financial records and issue an opinion regarding the financial statements of the company.
Internal audits are conducted throughout the year, while external auditors conduct a single annual audit. If a client is publicly-held, external auditors will also provide review services three times per year.
In short, the two functions share one word in their names, but are otherwise quite different. Larger organizations typically have both functions, thereby ensuring that their records, processes, and financial statements are closely examined at regular intervals.