Hyphens

What is it?

U+002D - HYPHEN-MINUS

Every keyboard I've ever seen has a hyphen key on it, so this shouldn't be a problem.

To link words together, as in "wolf-person", or
to add prefixes to capitalized words, as in "un-American", or
to break words (at syllable boundaries) over multiple lines, in a block of justified text.

U+2013 – EN DASH

With a compose key (Linux): "Compose", then "--."
MacOS: Option + "-"
Windows: Alt + "0150" (and they say Windows is user-friendly)
- I've heard people recommend WinCompose to get the functionality of a Compose key on Windows.

To specify ranges, as in "1974–1976", or
to link a more than one word to another word, as in "computer science–related", or
under some style guides, to link clauses together, as in "But future facts had to be solved, which undoubtedly would be treated with more comparative reverence than heretofore, by him who suffered severely – yea, acutely – from the blow struck him on the eve of aspiration and achievement."
- Note the spaces around the dash in this case.

U+2014 — EM DASH

Under some style guides, to link clauses together, as in "The Howden Estate, on which are elegantly formed two buildings of note—namely, Blandford Castle and Lauderdale Lodge, both exquisite constructions of architecture and skilled workmanship, and occupied respectively by Sir Sydney Hector and Admiral Charles Depew—lies chiefly around the south-west of Yorkshire, and is not quite so desirable or adapted for agriculture as the two first mentioned, being mostly rented for grazing purposes by the numerous and varied owners of its rugged plots."

U+2212 − MINUS SIGN

(Everybody seems to think a hyphen works fine for this :'( The hyphen's name is even "hyphen-minus"!)

Linux: Left control + Shift + U, then 2212. (No, I don't know why there's not a compose sequence for it.)
MacOS: this nonsense
Windows: Who Knows

When writing negative numbers, as in "−42", and in subtraction expressions, as in "50−8". (People tend to use the hyphen for this instead.)