A contraction is a shortened form of a word (or group of words) that leaves out certain letters or sounds. In most contractions, an apostrophe represents the missing letters. The most common contractions are made up of verbs, auxiliaries, or modals attached to other words, for example –
He would = He’d. I have = I’ve. They are = They’re. You cannot = You can’t.
Contractions are commonly used in speech, but in writing they’re usually considered to be relatively casual. If you’re writing something very formal, you may want to avoid using them.