in the beginning, operating systems on personal computers (or pc) did not have the concept of access control. there were no "users", everyone could do everything. modern operating systems have the concept of access control. there can be more than one user and some users may have more "privileges" than others. certain users may only have access to certain files and are unable to access others. other users may be able to modify or delete certain files. these privileges can be specified for each file.