The file creation mask is what determines the permissions a file will be assigned upon its creation. The mask restricts or masks permissions, thus determining the ultimate permission a file or directory will be given. If no mask were present directories would be created with 777 (rwxrwxrwx) permissions and files would be created with 666 (rw-rw-rw-) permissions. The mask can and is typically set by the system administrator, but it can be overridden on a per account basis by including a umask statement in your personal initialization files.
