Formal is talking to people you don’t know. For an example: Formal is talking to people like bosses, reports, speeches, or application. Informal is talking to people you know. For an example: Informal is talking to people like friends or family members.
Formal language is writing a unique letter to your boss or for an application. For an example: In Formal language, the grammar is more complex and its more specific.
Informal language is writing a casual letter to your friends or family member. For an example: In Informal language, the grammar is casual and friendly.
Formal and Informal language is the way people talk.