Define auditing and assurance engegament

Auditing (broadly defined) is a systematic process of objectively obtaining and evaluating evidence regarding assertions about economic actions and events to ascertain the degree of correspondence between those assertions and established criteria and communicate the results to interested users.
Attest services occur when a practitioner issues a report on subject matter, or an assertion about subject matter, that is the responsibility of another party.
Assurance services are independent professional services that improve the quality of information, or its context, for decision makers.