In many languages, a hyphen (-) is a single symbol, while a dash (--) is a double symbol. Dashes are primarily used by publishing experts, such as newspaper and book editors, as a substitute for a semi-colon (;) to get a more fluid feel of a text. To make matters more confusing, people typically call a hyphen (-) within a URL as "dash" when saying a URL out loud. Thus, www.hs-heilbronn.de would be pronounced "www DOT hs DASH heilbronn DOT de" because it is faster to say than "hyphen".
An example:
They can also be used around parenthetical statements–-such as this one-–in place of the em dashes preferred by some publishers, particularly where short columns are used, since em dashes can look awkward at the end of a line.
More detailed information: Dash in Wikipedia