Common English Contraction

A contraction is a shortened version of the written and spoken forms of a word, syllable, or word group, created by omission of internal letters.

Example:

Let's go! --> Let us go!
What's this? --> What is this?

Positive Contraction:

I'm  --> I am
I'll --> I will
I'd  --> I had / I would
I've --> I have

he's  --> he is
he'll --> he will
he'd  --> he had / he would

she's  --> she is
she'll --> she will
she'd  --> she had / she would

it's  --> it is
it'll --> it will
it'd  --> it had / it would

you're --> you are
you'll --> you will
you'd  --> you had / you would
you've  --> you have

what's --> what is
where's --> where is
when's --> when is
why's --> why is
how's --> how is

what're --> what are
where're --> where re
when're --> when are
why're --> why are
how're --> how are

what'll --> what will
where'll --> where will
when'll --> when will
why'll --> why will
how'll --> how will

Negative Contraction:

aren't    --> are not
can't     --> can not
couldn't  --> could not
didn't    --> did not
doesn't   --> does not
don't     --> do not
hadn't    --> had not
hasn't    --> has not
isn't     --> is not
mustn't   --> must not
needn't   --> need not
shouldn't --> should not
wasn't    --> was not
weren't   --> were not
won't     --> will not
wouldn't  --> would not

References:
http://en.wikipedia.org/wiki/Contraction_(grammar)
http://esl.about.com/od/grammarintermediate/a/contractions.htm