Access Control Lists are critical for defining and managing security in networked systems, databases, and file systems. An ACL contains a list of permissions associated with an object, such as a file, directory, or network resource. Each entry in the ACL specifies a subject (like a user or role) and the type of access granted to that subject, which could include permissions like read, write, execute, or delete. The primary purpose of ACLs is to ensure that only authorized users can perform specific actions on the resources, thus protecting data integrity and confidentiality.