The most overused and underappreciated punctuation mark utilized in the writing of the English language. It is used to indicate the omission of a letter (elision) or letters in a word, to connect words pronounced successively (contraction), or to indicate possession.
Alternatively, and much less contemporary, is the use of the term "apostrophe" to indicate a short address by a character in a play or poem to an absent party or an inanimate object. The apostrophe in this sense allows the audience a look into the speaker's thoughts toward a person or object that cannot respond (similar to a soliloquy).
Correct: It's movie night.
Incorrect: Its movie night.
Incorrect: Of it's own free will.
Correct: Of its own free will.
He's not the type to say "Don't do it."
Ned is a ne'er-do-well.
"O'er the fruited plain..."
That is Jack's grammar book.
Jesus' life lasted 33 years.
-in a literary sense...-
Rhodora! if the sages ask thee why
This charm is wasted
on the earth and sky,
Tell them, dear, that if eyes were made for seeing,
Then Beauty is its own excuse for being:
Why thou wert there, O rival of the rose!
I never thought to ask, I never knew:
But, in my simple ignorance, suppose
The self-same Power that brought me there brought you.
by Emerson (1839)