There are three main types of audits: external audits, internal audits, and Internal Revenue Service (IRS) audits.
External audits are commonly performed by Certified Public Accounting (CPA) firms and result in an auditor's opinion which is included in the audit report.
An unqualified, or clean, audit opinion means that the auditor has not identified any material misstatement as a result of his or her review of the financial statements.
External audits can include a review of both financial statements and a company's internal controls.
Internal audits serve as a managerial tool to make improvements to processes and internal controls.