The Hyphen: -
What she lacks in width, she makes up for in her ability to sew words together. Hyphens are placed between words, and never padded with spaces. There are many rules when a hyphen is fitting. The most common being between two or more adjectives when they come before a noun, forming a singular descriptor:
The well-worn couch.
You can validate that the two adjectives above are acting as a singular descriptor as none of them alone properly describe the couch: “friendly-couch”, or “looking-couch.”
Another use of the hyphen is between numbers:
I’m twenty-eight? Where’d the years go.
The En Dash: –
The middle child of the family. She’s wider than the hyphen, but not as bold as her older sister. The en dash has the simplest usage—between dates and number ranges:
On a bad day, I can eat 2–3 burritos.
I’ll be traveling the west coast February–March 2013.
The Em Dash: —
The widest stroke of them all. To quote The Elements of Style:
“A [em] dash is a mark of separation stronger than a comma, less formal than a colon, and more relaxed than parentheses.”
An example usage:
When changing a tire—a task eventually faced by all drivers—it’s easiest to loosen the lug nuts while the wheel rests on the ground.
A common and incorrect replacement for the em dash is placing two hyphens next to each other: --. Thankfully, Medium detects this usage and automatically replaces them with the correct em dash.
There’s a scattering of opinion if em dashes, (named em, as they are typically the width of the ‘m’ character) should be surrounded by a single space. A quick, and not always correct, rule of thumb: American style guides typically prefer unspaced em dashes, whereas Canadian, and UK style guides lean towards spaced em dashes.
There are many other, and more particular uses for dashes and hyphens outlined on wikipedia.