Periods end a sentence. 

Simple. Too simple? Fine. 

They end a complete sentence.

They also end abbreviations.

Example: Mr., Sat., Oct.

*Helpful Hint: When ending a sentence with an abbreviation, only use one period, not two.

Experienced writers are yelling at their screens right now, shouting something about fragments not being complete sentences. 

Listen, the writing world is changing. Within reason, sentence fragments can add to the emotional state of your characters, or draw your readers into a suspense filled state. Why would you ruin that with perfect, complete sentences all the time?

Continuing on. (This sentence wouldn't have the same impact if I'd written "I'm continuing on," right? Of course it's me, duh!)

To clarify: Periods end an idea. If you can't figure out if you need a period or a coma, you're in luck. 

Next up: Comas!