The word hyphen comes from the Greek for together, which reflects the hyphen’s function as a connector. Dash is descriptive: it’s a bold stroke of punctuation, which can hive things off from each other as well as connect otherwise disparate elements.
The hyphen as connector
English, like German, likes to combine two or more words into one. The Germans just shove them all together, stringing a series of words into one long chain (Rindfleischetikettierungsüberwachungsaufgabenübertragungsgesetz apparently being the longest; it means ‘beef-labelling supervision duties delegation law’, formerly in the statute book of the German state of Mecklenburg-Vorpommern, but now since repealed).
In English, the tendency is to hyphenate compounds, at least initially. When people started to talk about holders of shares a lot, it made sense to make a compound: share-holder. Later (as often happens), the hyphen disappeared and left us with shareholder. Hyphens are also disappearing from words like vice-president, but leaving two words (vice president) rather than one. Fowler’s Modern English Usage says this is actually OK – but doesn’t vice chair sound a bit naughty?
The hyphen tends to stay, however, where its disappearance would leave a pile-up of letters that don’t naturally go together in English or which suggest a weird pronunciation: security-holder and loop-hole are preferable to their hyphen-less versions. Co-operative is better than cooperative for that reason, but unco-operative does admittedly look a bit odd (and gets a red wavy line under it when typed on the screen – not that one should pay unquestioning heed to that). The New Yorker has tried for years to get people to write coöperative, but nobody is buying it.
Hyphens are usual in compound adjectives: pea-green boat, 20-year-old whisky, end-of-year review, red-hot chili peppers (a shame it’s too late to educate Anthony Kiedis about the hyphen). Not using a hyphen can lead to confusion: are you getting extra marital sex or extra-marital sex? Where the first element in a compound adjective is an adverb, the hyphen sometimes disappears: a well-deserved vacation BUT (probably) a soundly argued factum. The rules on that last point are murky; be guided by common-sense.
I’m of two minds about constructions like second- and third-hand information. Fowler’s Modern English Usage (the Bible for these things) suggests that the first hyphen can go (and may be better from a stylistic perspective than the alternative second-hand and third-hand information). There may be times when you’d want to keep it, though, for the sake of clarity: the meaning changes if you drop the first hyphen in This book is intended for the ill- and well-educated.
Another common use of the hyphen is for compound surnames. This used to be a posh thing, where the surname of a wife or relation was added in order to inherit that person’s property (the most elaborate example is Temple-Nugent-Brydges-Chandos-Grenville). It’s now an egalitarian thing, so kids get both their parents’ names. Sometimes the hyphen is omitted in these formations, which can lead to confusion: is that a middle name or part of the family name?
The dash has many uses, and as many—or more—misuses. Chief among the uses is to add a parenthetical phrase in the middle of a sentence, by way of explanation or for emphasis. If you hadn’t noticed, there is an example of in the opening sentence of this section. You can also do it at the end of a sentence, for emphasis—as I’ve just done now. Go easy, though: too much emphasis de-emphasises.
A bit of typography
The hyphen is one short stroke, like so: –
There are two types of dash: the ’em’ dash (—)and the ‘en’ (–), so-called by printers because they are the length of the bits of metal type used for printing the letters M and N respectively. (They are also called the long and the short dash.)
For purists, the en dash is used to separate date ranges (1837–1901) and connected or contrasted pairs of words (the North–South divide). The em dash is reserved for breaks in sentences. There should be no space on either side of a dash.
In the days of the manual typewriter, the em dash was rendered with three hyphens (—), the en with two (–). Typography nerds decry the continuation of this practice in the digital era, but you may have to resort to the ‘special characters’ menu to get the em dash, or figure out some combination of option and shift keys (which eludes me). In common usage, most people use a hyphen for date ranges and word-pairs, an en dash for sentence-breaks (your computer will generally convert two hyphens into the latter).
Next: who and whom