An audit confirms that the financial statements of the client are fair and accurate. It is not always possible to provide a 100% assurance, but the essence of an audit is to give a reasonable level of assurance that there are no errors in the financial statements and that they conform to generally accepted accounting principles (GAAP).
Some of the approaches used in audits are to carry out enquiries both within and outside of the company, review and assess internal controls, assets counting, and a thorough review of documents.
Reviews do not provide as much assurance as audits do. Though the essence of an audit is to provide assurance that financial statements are fair and absent of any financial misstatements, reviews provide limited assurance that the financial statements are in accordance with GAAP. This limited type of assurance is usually referred to as negative assurance.