Figuring out where and when to use a comma can be tricky. They're pesky little buggers, aren't they? One situation when you sometimes use one (and sometimes not) is with appositives and appositive phrases.
An appositive is a noun that describes another noun, such as:
My father, Mike, is retired.
When the movie was over I called my mom, Laurie, to pick me up.
An appositive phrase would be:
Stephen King, a New York Times bestselling author, has millions of fans.
Being a huge supporter of the cause, Rachel donated thousands of dollars.
So, when do you use commas in these situations?
When the information is necessary for the meaning of the sentence you do NOT need to separate it with a comma. If the information is extra and nonessential, you DO separate it with commas.
My friend Sarah just got married.
I presumably have more than one friend, so her name (Sarah) is essential information and therefore is NOT separated by commas.
Example of when you do use commas:
At our high school, the principal, Mr. Torreli, has a very strict policy on tardiness.
There is only one principal at the school, so the extra information (his name) is nonessential and therefore set apart with commas.
In a situation where you have both, it might look like this:
My uncle David brought his wife, Shannon, to my graduation.
I have more than one uncle so his name is essential (not set apart) but he only has one wife, so her name isn't necessary and IS set apart.
What are some punctuation rules that confuse (or just plain irritate) you? :)