Years ago I read something about how they can compare one's writing and style and match it with other stuff that is “anonymous”. I often write stuff on our “anonymous” Free Republic and think “Oh - they'll be able to pick me out from that sentence.”
That's okay - keeps me polite. And legal.
An em dash is a long dash. It functions more like a strong break, where a regular dash or hyphen connects compound words.
Strictly speaking, the "em-dash" is a horizontal line whose length is the width of the letter "M" in standard typeface. Similarly, the "en-dash" has length the width of the letter "N" in standard typeface. But it varies, and em-dashes tend to get pretty long for stylistic reasons.
The usage dates back to physical typesetting and has been carried forward into computer typefaces. Because different fonts have different widths for the "M" and "N" characters, the lengths of the various dashs (including the so-called "medium-en-dash") also vary with the font.
There is also the "digit-dash" (also "figure-dash") which is the width of the number "0" in the standard typeface.
Another interesting point is whether a blank/space is placed before and after a given dash. Generally not for the en-dash, and sometimes for the em-dash. Again, it varies with style.