In spoken English we use the short form a lot. Instead of "I am" we say, "I'm". "You are" becomes "you're". "Did not" - "didn't" etc. We also use these short forms in informal written English. When we write the short form, we use an apostrophe (') for the missing letter(s). This site gives you a good, quick reference for the short forms of the auxiliary verbs 'to be', 'to do' and 'to have'.
Short forms (contracted forms) in English