Hyphens and Dashes
A hyphen connects (or links).
A dash separates.
A hyphen is one touch of that line key near the upper-right corner of your keyboard (the hyphen key).
A dash is two touches of that line key near the upper-right corner of your keyboard (the hyphen key).
Something else you need to know: English is a language that, over time, moves together, like hitting the close-the-doors button on the elevator.
One hundred years ago, you would usually spell the words this way: sand box. See the space?
Fifty years ago, the words were used together enough that they were often merged, brought together, linked, conglomerated with a hyphen: sand-box.
Now, we usually spell it this way, as one word: sandbox.
So we use a hyphen to bring words together. Usually, these are words brought together to become compound adjectives: The bear-like man was very strong and hirsute. Sometimes, a hyphen is used in a noun: Spider-Man.
A dash is made by connecting hyphens in order to construct a mark that pushes away rather than brings together. Its most common use these days is in the middle of a sentence when you are inserting information and it feels like too much for commas to handle.
Dashes—which have no space between or around them (if you can help it, and in Google docs you can’t; Google docs keep a space between the hyphens) and allow you to fill your sentence to bursting with information—are very useful punctuation marks. Please, please, please stop using a hyphen when you mean to separate. Even more please, stop putting a space after a hyphen and declaring to the world, “There, now I have a dash.” No, you don’t. Remember what I said about the English language and elevator doors? Well, a dash does the opposite to parts of a sentence—it sticks a bar between the doors and forces them to remain open.