Access Control Lists allow you to restrict access to specific wiki pages to individual users, or groups (teams) of users. Each page (or a hierarchy of pagers) can be restricted to read only for users, or access can be revoked entirely so specific users cannot read the page(s) at all.
Access Control also gives you the ability to create read-only (guest) users. Guest users can only read the wiki, they cannot change any content. You have the ability to grant guest users the ability to comment on wiki pages and view the user directory.