An ELT Glossary : Contractions

English contractions are formed by "blending" two words, of which at least one will be a grammatical item (an auxiliary verb, the copula, a pronoun, the negative particle etc)  into one. Examples :

I am   becomes  I'm
They would  becomes  they'd
Paul is  becomes  Paul's
Let us  becomes  let's
Does not  becomes  doesn't
Will not  becomes  won't

As these examples show, contractions involve elision (the dropping of one or more sounds) but may also involve other features - such as change in vowel quality in will not/won't or the non-standard is not/ain't.