The en dash is shorter than the em dash.
There is no space before or after either dash.
The en dash is approximately the length of the letter n, and the em dash the length of the letter m.
The shorter en dash (–) is used to mark ranges.
The longer em dash (—) is used to separate extra information or mark a break in a sentence.
They are usually found above the P on a keyboard.
When do you use these dashes?

How Do You Make These Dashes on a Computer?
For Windows Users:
How to insert Unicode characters in Word?
To insert an ASCII or Unicode character, press and hold down ALT while typing the character code.
To insert the em dash (—) symbol, press and hold down ALT while typing 0151 on the numeric keypad.
To insert the en dash (-) symbol, press and hold down ALT while typing 0150 on the numeric keypad.
You must use the numeric keypad to type the numbers and not the keyboard.
For Mac Users:
To type an em dash, hold down the Shift and Option keys and press the Minus key. Alternatively, an em dash can be typed with a double hyphen by pressing the Hyphen key twice and then press Space.
To type an en dash, hold down the Option key and press the Minus key.
Hyphen on the Keyboard
Computer users often use the hyphen above the “P” as an en dash, but printers and typesetters use the en dash.
All of these Question of the Day things–or is it Question-of-the-Day things–are very helpful to me. Thanks.
You’re very welcome.
They help me, too.
The questions are usually ones that have come up for me in the past week.
I research the answers and then share them with you.
Thanks, Nancy.
Truthfully, I figured if I was such a dunce about this stuff, most of my readers would be too. Now there’s no excuse—right? Seriously, thanks.