To provide an option by short name, use a short form: a single dash followed by the option’s short name (e.g. ‘-h’). To provide an option by long name, use a long form: two dashes followed by the option’s long name (e.g. ‘--help’).
Short names typically consist of a single character. They are concise but more difficult to remember. Long names can be whole words, or even word sequences (usually separated by dashes). They are easier to remember but longer to type on the command-line.
You should be aware of the fact that in a clonified application, neither short names nor long ones are restricted in length. In fact, it would be perfectly possible to have an option’s short name longer than the corresponding long one, although that would make little sense. The only real difference is whether you use one or two dashes. Some other differences also come into play when an option expects an argument (see Option Arguments) though (it would be no fun otherwise).