One em
is the maximum character-width of the current font (the width of an upper-case M). One ex
is about half the height of the current font (the height of the lower-case x).
Sadly, if you aren't using a monospaced font (one where all the letters take up the same amount of horizontal space, such as Courier), the width of a certain combination of letters will not be the same as every other combination of as many letters.
Example:
Stack Overflow does not use a monospaced font for its comments, hence these following lines are of different width, though containing the same amount of letters:
iiiiiiiii
MMMMMMMMM
However, if I write in a monospaced font, they do occupy the same horizontal width:
iiiiiiiii
MMMMMMMMM