Both speech and writing can be formal or informal. Whether language is formal or informal depends on who our readers and listeners are, for what purpose we use the language and the situation in which we use it. For example, an official notice would be written in formal English, whereas the same information would be more commonly written informally by an individual in a less public situation:
Customers are reminded that parking is not permitted in front of this entrance. Thank you.
Please don’t park in front of this gate. Thanks.
A lot of writing these days is informal. Common examples of such writing are emails, text messages, advertisements, postcards, notes and a lot of the language used in internet discourse. Informal writing creates a direct dialogue with the reader.
Some magazines and newspapers also include a lot of informal writing. Informal writing contains forms which are more commonly found in speaking, such as contractions, personal pronouns, ellipsis, spoken discourse markers and question tags:
[article in a magazine about an interview with a pop star who is marketing a new perfume]
Well, how long was it, do you think, before we talked about it? Exactly. You’ve got it. Over two minutes.