Beloved by Scrabble fans, em (/εm/) can mean the letter M or a unit of measurement in typography – hence em dash (one of which appeared just there, before hence). Occasionally em appears as an interjection, an onomatopoeic murmur equivalent to erm or um, while in HTML it italicizes text.
Em with a capital E is a popular nickname for Emma, Emily and Emmanuel, while the all-capital EM is a standard abbreviation of electromagnetic and electron microscope, among other things. Em- is a common prefix, found in words such as embark, embed, embody, emboss, embrace, and embroil.
‘Em with a preceding apostrophe is a common informal variant of them, often seen and heard in colloquial expressions such as “Set ‘em up Joe”, “Give ‘em hell”, “Up an’ at ‘em”, “Stick ‘em up”, “Ride ‘em cowboy”, “Use ‘em or lose ‘em”, “Texas hold ‘em”, “Read ‘em and weep”, and “Some Mothers Do ‘Ave ‘Em”. In spoken English the ‘em is usually unstressed (/əm/), and is occasionally absorbed into the associated verb, such as in the video game title “Duke Nukem”.
What is curious about ‘em is that contrary to popular belief, the apostrophe does not denote the missing letters “th”. In Middle English ‘em was an alternative form of hem (an old pronoun, not the verb, the clothing term or the throat-clearing interjection), which was later displaced by them. So ‘em does not derive from them; in fact it precedes it.