The mark you use makes a difference.
|Think of punctuation as street signs for the reader: using the right one is crucial to get them where you want them.|
The period, or full stop if you're British, comes at the end of a declarative sentence:
I'll get some coffee.
The exception to this rule is when the sentence is in a quotation, with the attribution following:
"I'll get some coffee," she said.
The comma separates two or more items in a series, or information that is not critical:
The list calls for bread, eggs, milk, and cereal.
Her leg, which she'd injured snowboarding, was bothering her.
The semicolon goes before a conjunctive adverb, or between items in a series that contain their own commas:
Your concern is touching; however, I think I can do this.
The recipe calls for two large yellow onions, thickly sliced; grated fresh coconut, or dried unsweetened coconut; and lemon juice.
The colon is for introducing a list:
Here are some examples of what I'm talking about:
Parentheses hold clarifying information:
My sister (who I hadn't seen or heard from in five years) asked me why I didn't send her an invitation.
The dash can serve as a dramatic parenthesis:
The engine - temperamental in the best of conditions - conked out while I was trying to drive home in a blizzard.
The apostrophe indicates contraction or possession:
It's nice to see you again.
Bethany's car was in the shop again.
The placement of the apostrophe indicates the amount of whatever or whoever gets the apostrophe: boys' bikes indicates that each bike belongs to a different boy, while boy's bikes indicates that all of the bikes in question belong to the same boy.
Quotation marks are exactly what they say they are, but there are some specific rules with them: periods and commas always go inside quotation marks, while colons and semicolons go outside, unless they are part of the quote. Question marks and exclamation marks go inside the quotation only if they're part of the quote.
The ellipsis indicates the omission of quoted material; or, occasionally, a dramatic pause:
"Amazing... unbelievable... "
"This could be trouble... "
Notice that only three points make up the ellipsis, and that it's always followed by a space.
I hope this helps you to improve your writing. It has with me: I found out I was misusing some punctuation marks in my own writing.