From Carol Ann Tomlinson's The Differentiated Classroom: Responding to the Needs of All Learners (1999):

In the differentiated classrooms, teachers begin where students are, not the front of a curriculum guide. They accept and build upon the premise that learners differ in important ways. Thus, they also accept and act on the premise that teachers must be ready to engage students in instruction through different learning modalities, by appealing to differing interests, and by using varied rates of instruction along with varied degrees of complexity. In differentiated classrooms, teachers ensure that a student competes against himself as he grows and develops more than he competes against other students.

In differentiated classrooms, teachers provide specific ways for each individual to learn as deeply as possible and as quickly as possible, without assuming one student's road map for learning is identical to anyone else's. The teachers believe that students should be held to high standards. They work diligently to ensure that struggling, advanced, and in-between students think and work harder than they meant to; achieve more than they thought they could; and come to believe that learning involves effort, risk, and personal triumph. These teachers also work to ensure that each student consistently experiences the reality that success is likely to follow hard work.

Teachers in differentiated classes use time flexibly, call upon a range of instructional strategies, and become partners with their students to see that both what is learned and the learning environment are shaped to the learner. They do not force-fit learners into a standard mold. You might say these teachers are students of their students. These teachers also are artists who use the tools of their craft to address students' needs. They do not reach for standardized, mass-produced instruction assumed to be a good fit for all students because they recognize the students are individuals.

Teachers in differentiated classrooms begin with a clear and solid sense of what constitutes powerful curriculum and engaging instruction. Then they ask what it will take to modify that instruction so that each leaner comes away with understandings and skills that offer guidance to the next phase of learning. Essentially, teachers in differentiated classrooms accept, embrace, and plan for the fact that learners bring many commonalities to school, but that learners also bring the essential differences that make them individuals. Teachers can allow for this reality in many ways to make classrooms a good fit for each individual. (p. 2)