You don't use an apostrophe if you're referring to the plural form of a noun, like you did in the topic title.
You can use it if you want to contract two words ("do not" can be contracted to "don't" if you're not writing a formal document)
Or you can use it to show possession, adding an apostrophe followed by a lowercase S to a singular noun or proper noun, or a lowercase S followed by an apostrophe if referring to more than one noun or proper noun.
If you're still reading this, I commend you.
Although an average quotation mark is typically used, you can use apostrophes for quotation. You can see this happen in the unabridged Lord of the Rings books by J.R.R. Tolkien.
Damn. I just gave an entire lesson on a single punctuation mark.
Heil Spellcheck, gentlemen.
19.5: Give a grammar lesson on an online forum.
Warning - while you were typing 15 new replies have been posted. You may wish to review your post. Damn.
