For me, that mark is the semicolon. It's that thing you use on your computer to make a winking smiley. It shares a key with the colon ( : ), and it's what you get if you forget to hold down the shift key.
The semicolon's function is an emphatic comma. If you're writing out a list, and some of the things in the list need further separation, you use a semicolon to separate the main items.
At the grocery store, she bought eggs; milk; butter; a salad with olives, spinach, and feta cheese; and some fresh fish for dinner.Its main purpose, though, is to connect sentences which would be run-ons or a comma splice without the semicolon. If you have two whole sentences which are related, you can join them with a semicolon:
I'm a night owl; I'm rarely in bed before 1 AM.You also use it before however, moreover, thus, consequently, nevertheless, and therefore. Again, an example:
I rarely get enough sleep; consequently, I have quite the caffeine addiction.The thing about semicolons is, you can usually leave them out, except in complex lists, as outlined above. The purpose of a semicolon is entirely that of flow. Were I to write the above sentence as two, it might read as choppy or abrupt, but it would still be correct. The notion of a punctuation mark's primary purpose being to vary sentence lengths and aid the flow of the words appeals to me very much.
I distinctly remember never using semicolons until I learned them in high school, and then squashing them in wherever they would fit, once I had the hang of them. It was like the invention of the cordless phone: freeing and fun and overused for everyone but me.
I sprinkle them much more conservatively these days. You can overuse semicolons, and they're considered passé by some publishing circles. I'll continue to use them until an editor tells me to stop, but I'm not going to waste them on trivial sentences in the meantime.