What are the major differences between a formal and informal essay?

Formal essays are used with proper English, and don't use pronouns or reference yourself

Some key differences:

Formal essays usually have a formal manner in them. If you were to speak the essay out loud, then your audience would probably get the feeling that this was a very important essay, and seriousness would probably be found.

In an informal essay, there would often be slangs, unprofessional use of wording, that kind of stuff. When you read it out loud, you might provide the audience with a small laugh of sorts.