Topic 1: 's: What is it for?
When you use an apostrophe s (ie a 's) it usually signifies possession. In some cases 's can be used as a contraction.
Example (posession): The cat's food was not acceptable to her delicate pallet.
The cat possesses the sensitive pallet.
Example (contraction): It's annoying that she only eats the most expensive organic canned food available.
It is annoying that the cat does this.
However, an exception to the use of 's to indicate possession is when the object is owned by more than one person.
Example: The students' drinks were the cheapest on the bar menu.
There is more than one owner, so 's changed to s'.
Topic 2: What is the difference between your and you're?
People get these two confused, possibly because of the widespread use of abbreviations in online chats, or possibly because the general population is idiotic.
You're is a contraction. It's short for you are.
Example: You're an asshole, and I love you. (P!nk)
P!nk is saying her husband is an asshole.
Your is a word indicating possession of something.
Example: Your face looks like a butt.
I am saying that the face that you possess looks like a buttocks.
BONUS: Yore is a word referring to the past. It's hilarious to use this word instead of one of the above to confuse the hell out of people.
Topic 4: There, their, and they're- are they interchangeable? (No.)
There refers to a place other than here.
Example: You can set the pile of exams to be graded over there, my desk isn't sturdy enough to support them.
I'm indicating that the exams should be set down somewhere other than my desk.
Their is used to describe something that is possessed by multiple people.
Example: After her angry outburst, their mouths hung open for a few seconds in silence.
They're is a contraction of the words they are.
Example: They're a bunch of jerks, which is why I snapped at them.
They are poop heads, and deserved what they got.
Topic 3: To, too and two
To is a word that you should not google because, inexplicably, pictures of asses in teeny thongs pop up.
Example: They delivered the dishwasher right to the door.
Too means "also."
Example: My friend doing her PhD at BU said she's drinking alone too!
My friend is also drinking alone on a Friday night.
Two is a freaking number.
Example: One, two, three.
Watch Sesame Street.
I hope this entry helps everyone brush up on their English. I will not be accepting any more emails, especiallyvia dating websites, that ignore these rules.