In order to be able to write an essay on the differences between formal and informal writing, one needs to know how to identify and define both informal and formal writing. The best way to explain the differences is to provide examples.

In formal writing, the language is elevated and does not mirror normal conversational dialogue (which informal writing does). Formal writing does not use colloquial words (words used in ordinary or familiar language--which informal writing does)). Formal writing avoids contractions (which are normal in every day conversation and can be found in informal writing).

Example of formal writing: The canine which, was picked up by the Humane Society, was euthanized given that the animal was unable to be controlled and attacked the handler.

Example of informal writing: You know that dog that the cops picked up? Yeah, it was "put down" 'cause it bit some dude. He just couldn't handle the mutt.

Think of it this way, informal writing can be seen if a person was to write down a conversation between you and a friend. You would use slang (colloquial words), contractions, and the first person ("I").