ACL in C:\Winnt

Can anyone give me a solid explanation of what the .acl files are doing in my C:\Winnt?
I am running NT4 on my workstations, and I notice that there are numerous acl files here, taking up disk space. They are prefixed by a login name, such as JoeD.acl. They are small, but numberous, and about 36k. I believe that they relate to login ID’s and access control lists and permissions. A more detailed description and whether they can be deleted is appreciated.