What is differentiated instruction?

The development of differentiated instruction over the years is outlined in a good article.

Washburne takes the reader through a short history of reform efforts aimed at making education more individualized, beginning with the efforts of charismatic educator Preston Search, of Pueblo, Colo., and proceeding through the work of Frederic Burk, who "started a movement to make textbooks self-instructive and enable children, systematically, to progress according to their own ability." From the Project Method to ability grouping, Washburne's history shows how evolving understanding of learning and development shapes educators' efforts to meet the needs of all students.

Read the full article here. Bold in quote for my emphasis.