Assurance engagement is an engagement performed by a practitioner to enable himself to express an opinion about the measurement of subject matter against a criteria. Practitioner’s expression of opinion raise the confidence of the users over the measurement of subject matter subject to the criteria applicable under consideration.
To summarize the above formal definition, read through the following example to understand how assurance engagement is performed and how it enhances the confidence of users
Practitioner (auditor) is appointed by the shareholders (users) to express his opinion (assurance) in the form of a report over the recognition, measurement, presentation and disclosures i.e. financial statements (subject matter information) made by the management (responsible party) are actually in accordance with IFRSs (criteria) and the actual financial position, performance and changes in cash flow (subject matter). From the definition of assurance engagement and also from the above example we understood that not every engagement undertaken by the practitioner amounts to assurance engagement. For an engagement to be assurance engagement it must have the following FIVE essentials or elements (also known as essentials or elements of assurance engagements):

thanks with simple explanations to undestand…..
What is the difference between statutory and non-statutory assurance engagements?
thanx for an excellent explanation. very simple to understand