Truth #1: A hyphen separates parts of words. You can even connect words with them to function adjectively. Teddy wants to re-think his career choice. He's in couldn't-care-less mode.
Truth #2: A dash separates parts of sentences, usually for emphasis. Baby, that chili is delicious--I'm talking, the best ever!
Truth #3: You usually have to key in two hyphens to make a dash. Most word processing programs then join them to make one longer line, but on blogger this is a hyphen -, and this is a dash --.
Truth #4: Yes, I'm being totally serious.
It can be kind of confusing for the reader if the wrong one is used-see what I mean? It may bamboozle the reader in a gee--that--doesn't--seem--right sort of way. Mm hmm...
So I hope my in-your-face approach makes sense and that you've come to appreciate the unique distinctions between the humble--yet powerful!--hyphen and dash.