"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)