An em dash marks the dramatic entrance of another related element in the sentence, or an authoritative sectioning off of nonessential sentence elements.
Basically, commas, colons, and parentheses walk onto the stage peacefully. Em dashes run onto the stage with spirit fingers blazing. Because they like to make a scene, only use em dashes when you purposefully want to interrupt the flow of a statement and prepare the reader for something important, or if you want to firmly fence off supplementary segments from the rest of the sentence.
"All chickenburgers are created equal—at least that's what I thought before I saw Food, Inc., a documentary about America's corporate controlled food industry."
In this example, the em dash is used to show a sudden change in thought. That is, the speaker originally didn't put much thought into where their food comes from. After they saw the documentary, their view changed. Their eating habits probably did, too.
"My boss—the most fashionable woman I've ever seen—broke both of her arms in a bar fight in Tijuana last weekend."
Although we have several questions about the speaker's boss after reading this sentence, the fact that she has an exceptional sense of style isn't essential to understanding it, so em dashes are used to quarantine that chunk of nonessential information.