A dash is longer than a hyphen. A lot of people confuse the two, usually using the hyphen in place of the dash.
A hyphen links up words: 'Scientists came to the extraordinary conclusion that the 52-year-old was formed from two non-identical twin girl embryos ...' A dash links up clauses (i.e. parts) within a sentence. It marks a pause or a sentence break.
Example 1: 'Tests carried out on the woman - known as Jane - showed she had two distinct types of DNA ...'
Example 2: 'Jane is a tetragametic chimera - someone whose body is made up of two genetically different lines of cells.'
Note that Example 1 contains two dashes while Example 2 has only one. The reason is that in Example 2, the end of the clause is also the end of the sentence. However, if the end of the clause is not the end of the sentence (see Example 1), then we should use a second dash. Just imagine it as a pair of brackets: when you open a bracket, you have to close it as well.