Dashes--typed as two unspaced hyphens--are like detour signs. They tell you that you're still on your way, but you'll have a delay. You place pertinent but not essential information or commentary inside dashes or parentheses.
- Dashes tell the reader additional information in an emphatic way. (Parentheses) can be used for this purpose but dashes are a more dramatic way to set off the information.)
My mother-in-law suddenly became ill--she turned red, clutched her throat, and began to wheeze--and all I could think was that I was killing her.
There was only one outstanding pianist--Gonzalez.
The cat--with its eerie light eyes and pure white coat barely visible--stayed in our memories like an apparition.
- Dashes set off an abrupt turn of thought within a sentence.
I served my new mother-in-law a cake with ground walnuts in it and then--good grief!--discovered she is deathly allergic to walnuts.
Tip: Dashes are not used frequently in formal writing.
Note: If you are using certain word processing apps, auto-format will change your two hyphens automatically into the longer dash. A hyphen is one (-); a dash is two (--).