"An informal writing style is a relaxed and colloquial way of writing standard English.
It is the style found in most personal e-mail and in some business
correspondence, nonfiction books of general interest, and
mass-circulation magazines. There is less distance between the writer
and the reader because the tone
is more personal than in a formal writing style. Contractions and
elliptical constructions are common. . . . An informal style
approximates the cadence and structure of spoken English while conforming to the grammatical conventions of written English."
(G. J. Alred, C. T. Brusaw, and W. E. Oliu, Handbook of Technical Writing, 9th ed. St. Martin's Press, 2008)