One textbook definition is as follows:
Internal controls encompass the plan of organization and all of the coordinated methods adopted within a business to safeguard its assets, check the accuracy and reliability of its accounting data, promote operational efficiency and encourage adherence to prescribed managerial policies. This definition recognizes that a system of internal control extends beyond those matters which relate directly to the functions of the accounting and financial departments.
Simply put, internal controls are anything we do to help us achieve our objectives. They are the policies, procedures, practices and organizational structures implemented in order to: