You may know your business in and out, but this may not be enough for other stakeholders such as banks, investors, regulatory bodies, etc. The external stakeholders may require a "certification" of your accounting information and assurance services may be required.
There are two major categories of assurance services - review and audit.
A review engagement is an assurance service based on analytical review and inquiries and provides a moderate level of "certification".
An audit engagement is an assurance service that provides a higher level of "certification" over the accounting information. However, the higher level of assurance requires more thorough work to assess the accuracy of accounting data provided.
Audit and review engagements can be performed for the Financial Statements taken as a whole or over specific accounting information only.
CPA Canada is the authority that provides the guidelines regarding the assurance services.