What's the difference
between a hyphen and a
dash? Aren't they the same?

Let's see what the Macquarie Dictionary says.

hyphen a short stroke [ - ] used to connect the parts of a compound word divided for any purpose. (Macquarie goes on to tell us, ‘Many compounds start out linked by a hyphen but as the compound form becomes more established there is a tendency for the hyphen to disappear.’) Examples include bestseller, groundbreaking and wheelbarrow.

dash a horizontal line [ – ] used in writing and printing as a mark of punctuation to indicate an abrupt break or pause in a sentence, to begin and end a parenthetic clause … and for other purposes.

Macquarie also gives another definition for dash:

dash a hasty stroke, especially of a pen.

Hyphens and dashes are a misused lot, and too often get used where a comma should be used, or even where a sentence should really end. (Go back to the intro sentence at the top of page 38, and see if this is your problem.) In lots of casual writing, even when it's intended for printing or publication, the poor old hyphen, or something like it, seems to have become the all-purpose punctuation mark. Why did I put a hyphen in all-purpose? Because I wanted those two words to be linked very clearly, as described in the definitions above.

Think of the sentence you're trying to write. You have two words side by side that you think might need a hyphen. Read the sentence, maybe a few times, maybe aloud. Would a hyphen make the meaning clearer? If it would, use one. If not, let the words remain two words.

But there are situations that always take hyphens:

• in double-barrelled words like double-barrelled, by-product, self-help, gluten-free, vice-president
• when a long word has to be divided at the end of a line. When you have to divide a word, put your hyphen between two syllables. For example, introduction, (not introdu-ction), esta-lish (not, estab-blish and never, never anything like establi-sh)
• in cases like re-cover, meaning ‘cover again' as opposed to ‘get over some illness'; and re-mark, meaning ‘mark again' as opposed to ‘speak out or comment'. In these cases the meaning is unclear without the hyphen
• in those contexts where you string several words together as if they really are to be considered one word, such as a never-to-be-repeated experience, his wake-up-late morning, a not-for-profit enterprise
• when you are writing double-barrelled numbers in words, for example twenty-five, sixty-two.

Here is an example of how not to use a hyphen:

This year will see bigger and better events for the festival-the 20th anniversary celebration!

In this example there is actually a new word. It's festival-the. Correct it by using a dash [ – ] and get:

This year will see bigger and better events for the festival — the 20th anniversary celebration!

What about this example?

If it's plastic-we have it.

Again, a new word has been created: plastic-we. The sign writer has actually connected the very words that were meant to be clearly separated. Use a dash instead:

If it's plastic – we have it.

Or simply a comma:

If it's plastic, we have it.

Here is a sentence that would be very much improved by the use of a few hyphens.

He says he's sick, but he just has first day of the week itis.

Where would you put the hyphens?

He says he's sick, but he just has first-day-of-the-week-itis.