The Cake Documentation Team has written an entire chapter about ACL. Here is what they cover:

1. Understanding How ACL Works
2. Defining Permissions: Cake's INI-based ACL
3. Defining Permissions: Cake's Database ACL
3.1. Getting Started
3.2. Creating Access Request Objects (AROs) and Access Control Objects (ACOs)
3.3. Assigning Permissions
4. Checking Permissions: The ACL Component

They have done a good job explaining the not so easy stuff :)

Update (2006-08-11): Link fixed.