What are the benefits of each, and when should I choose one over the other? Are there situations where these should be merged?
Do you have examples of common usages?
And what about MAC, where does that fit in?
RBAC (Role based access control) is based on defining a list of business roles, and adding each user in the system to one or more roles. Permissions and privileges are then granted to each role, and users receive them via their membership in the role (pretty much equivalent to a group). Applications will typically test the user for membership in a specific role, and grant or deny access based on that.
Discretionary Access Control (DAC) allows a user or administrator to define an Access Control List (ACL) on a specific resource (e.g. file, registry key, database table, OS object, etc), this List will contain entries (ACE) that define each user that has access to the resource, and what her privileges are for that resouce.
The main benefit of RBAC over DAC, is ease of management - in principle you have a very few roles, centrally administered, no matter how many users, and its just a question of granting each user the correct role; as opposed to DAC, where for each new user (or change in user, or deletion, etc), you have to go around to all the resources she needs access to and add them to the list.
On the other hand, DAC is often simpler, and generally more granular. Also, in the DAC model the data owner can decide who has access (if he has that permission on the data) and add or remove people from the list.
Very common example of DAC is the Windows file system. On the other hand, very common example of RBAC is the DAC on corporate file servers - anyone in the "Sales" ActiveDirectory group will have access to the \Sales\ shared folder. More commonly is the Administrators group in Windows.
MAC is Mandatory Access Control, this can be seen as a classification or privacy level. This is most often used in military systems, and back in the Mainframe days :). Not so much used anymore, though current OS's are implementing a flavor of this, such as Vista/Win7's Integrity Levels.
To sum up the differences: