If it is 'em with the apostrophe ( ') , then it is the contraction of 'them' as mentioned previously, an informal way to say 'them'.
If it is just 'em' it could be :
- em is a linear unit (1/6 inch) used in printing.
- an 'em quad' is a quad with a square body.
- EM as an abbreviation could refer to :
* Engineer of Mines
* enlisted man
Here you got 'em ;)