"Differentiation simply suggests that teachers have clear learning goals that are rich in meaning and provide various avenues and support systems to maximize that chance of each student succeeding with those rich and important goals" (Tomlinson, 2005, p. 14). Clearly, differentiation relies on a complex set of skills on the part of the teacher, a set of skills that can create an optimal environment that fosters maximum learning for all students -- within the same four walls of the same classroom. 

Source: Tomlinson, C. S. (2005). Middle Ground. Differentiating Instruction -- Why Bother?  9(1), p. 12-14.