In general, file and directory names should be composed only of letters of the alphabet, digits, ``.'''s, and ``_'''s. Be aware that files that begin with a ``.'' do not appear in the directory list unless a special flag has been set when doing the list command.
The period is often used to add a suffix on to a base filename. For example, the source code for C programs have a .c suffix added to them, e.g. prog.c. Separating a filename by a ``.'' is particularly useful when using wildcard selections (see section [*]).