We use formal language in situations that are serious or that involve people we don’t know well. Informal language is more commonly used in situations that are more relaxed and involve people we know well.
Formal language is more common when we write; informal language is more common when we speak. However, there are times where writing can be very informal, for example, when writing postcards or letters to friends, emails or text messages. There are also examples where spoken English can be very formal, for example, in a speech or a lecture. Most uses of English are neutral; that is, they are neither formal nor informal.
Formal language and informal language are associated with particular choices of grammar and vocabulary.