Discretionary Access Controls (DAC) define basic access control policies to objects. These are set at the discretion of the owner of the objects. For example, user and group ownership or file and directory permissions.
Mandatory Access Controls (MAC) are system-controlled access control policies where the system dictates and controls the level of access to an object, even a user created one. The administrator doesn't allow a user to grant less restrictive access controls to that object.
Mandatory Access Controls are considerably 'safer' than discretionary controls, but they are harder to implement and often require consideration tweaking to ensure all applications function correctly.
This was first published in September 2006