Formal and informal diction are two ways that authors can write. Formal diction is writing that is typically in 3rd person. The author does not use the words "I, we, our, my, us". Informal diction is similar to talking to a friend. It is typically written in 1st person.

For example:

Formal diction: The main character needs to make a decsion.

Informal diction: I think the main character needs to make a decision.

Additionally, formal diction does not typically use contractions, colloquialisms, or slang language. Also, words are not abbreviated in formal diction or writing (i.e. TV for television).