The em dash or em rule (—) is about the width of the letter m. An unspaced em dash can be used to indicate parenthetical information (ie as a substitute for commas, round brackets or en dashes):

The results of the experiment—which were circulated to all laboratories—were very interesting.

Many style guides (including this one) recommend a spaced en dash rather than an em dash, so check with your organisation or journal. Spaced en dashes also tend to be more common in digital publishing (see En dash).