Periods are used:
to mark the end of a sentence that is a complete statement:
My name’s Beth and I was 18 in July.
After leaving school, she went to work in an insurance company.
to mark the end of a group of words that don’t form a conventional sentence, so as to emphasize a statement:
I keep reliving that moment. Over and over again.
after some abbreviations—for example, etc., Dec., or p.m.:
The luncheon will be held on Thursday, Sept. 15, at the Waterfront Restaurant.
There’s a wide range of sandwiches, pies, cakes, etc., at very reasonable prices.
in website and e-mail addresses:
If an abbreviation with a period comes at the end of a sentence you don’t need to add another period:
Bring your own pens, pencils, rulers, etc.
See more advice about how to write abbreviations.