An audit is a thorough and systematic analysis of financial statements that is intended to provide a high level on confidence in the accuracy of said financial statements.  When a CPA performs an audit, they will issue a formal report in which they express their professional opinion on whether or not the financial statements are presented fairly and in compliance with the applicable reporting framework.  Audits are typically required when seeking financing, investors, when the governmental rule and regulation needs compliance, or when selling a business.  An audit is the highest level of assurance a CPA can provide those financial statements are free from material misstatement, but they do not provide absolute assurance.

The CPA performing an audit must demonstrate that their independence has not been impaired.

During the course of an audit the CPA will obtain an understanding of your company's internal controls and assess the risk of fraud. To do this the CPA will corroborate information presented in your company's financial statements by obtaining evidence in a variety of ways including inquiry, physical inspection, observation, examination, various analytical procedures, samples test, third-party confirmations...  

At the conclusion of the audit the CPA will provide a formal report that expresses their opinion on whether the financial statements are presented fairly and in all material aspects, in accordance with the applicable reporting framework.