Some of the most commonly used English contractions are aren't (are not), can't (can not), couldn't (could not), didn't (did not), doesn't (does not) and hasn't (has not). Contractions are formed by combining two words into one shortened word, with an apostrophe to taking the place of the missing letters.
The use of contractions is extremely commonplace in colloquial conversations, but it is considered more informal in writing. Whether or not using contractions in a piece of writing is appropriate varies based on the expected audience and desired tone of the piece. For example, it is appropriate to use contractions in a text message, but too informal to use them in an academic term paper.