See ACCC Online Disk Space Policy. A file name is a string of characters. UNIX does not give special relevance to periods or other characters, although some programs expect specific types of filenames. Each file has an associated owner and group. The owner is a logon account, and the group is a possibly empty group of logon accounts.
Permissions apply to directories as well as files.
Enter ls -l to find the permissions on the files and subdirectories in the current directory. Each line describes one file. The remaining nine characters are three triplets. The triplets give the read, write, and execute permissions for that file or directory for that file for, respectively, the file's owner, its group owner, and for the public. Thus, in this example, both files are owned by bobg and have the group comp as group owner.