Punctuation, for many — many — writers, represents a reprehensible beast, a monster with which grade school teachers tortured us, an entity of pure enmity — a featureless creature of ambiguous utility. Why do we need to learn this — asked we — Can't people still understand it without that comma?
If this is you, then behold: the em dash — the great weapon of Emily Dickinson's (right) affection, the very needle-tool of James Joyce's yarn spool!
Em dashes — which are the width of the letter 'm' and not found on your keyboard — "are used to set off or emphasize the content enclosed within dashes or the content that follows a dash," according to Purdue's excellent OWL site. In this definition, note carefully the uncertain term "to set off." This definition rightly implies that the dash — our mysterious friend — is capable of many things:
A dash can redefine something, such as Will Smith — the greatest hip-hop actor alive — or even Mark Wahlberg — the most handsome hip-hop actor.
Create Interjectory Clauses
It can add an emphasis at the end of a sentence — much like this! It can interrupt — like right now — a sentence. Hell — it can even start a sentence.
Replace a Colon
A dash can also begin lists — grocery lists, to-do lists, and even "people to avenge" lists
Replace a Semi-Colon
A dash can also join together two similar sentences — it can bridge a gap like a semicolon!
Interrupt a Thought or Sentence
A dash can help me change my mind: Shahrukh Khan is the sexiest — er, I mean funniest — actor in Bollywood Cinema.
Experimental writers of the Modernist Period — such as James Joyce — often felt compelled to stretch the bounds of the reader's ability to discern thought from dialogue. Joyce toyed with readers by starting paragraphs with an em dash to indicate some dialogue was about to follow, but didn't always tell the reader where the dialogue ended. For instance:
—It's a stinking mean thing, that's what it is, said Fleming in the corridor as the classes were passing out in file to the refectory, to pandy a fellow for what is not his fault. (A Portrait of the Artist as a Young Man, by James Joyce)
Note how he blends his dialogue with narrative. Of course, I'm not encouraging anyone to write like James Joyce, but I do think it is fun to stretch the bounds of a reader's ability to understand the written word.
In summary: When in doubt, use a dash!