Differentiation is the opposite of Lockstep Teaching. In Lockstep teaching all the learners are expected to be doing the same thing at and in the same time, and to achieve the same objectives to the same standards. If learning is differentiated, on the other hand, methods, objectives, activities, timing etc will be geared to the needs of the individual learners, so that in an extreme case each learner in the class might be working on something completely different.
This extreme type of differentiation (also called Individualised Learning) is rare however, because of the huge demands it places on the teacher - who ends up having to prepare 15 different lessons for a class rather than just one. However, given that most EFL classes have weaker or stronger students, and many may be officially "mixed ability", it is often essential to provide at least some degree of differentiation. There may be differing communicative needs that have to be catered for too.
Some simple examples:
- the T has done choral repetition of a model sentence and has moved on to individual repetition. She asks several strong Ls to repeat individually before calling on less able learners, so that they have the advantage of the extra models.
- the lesson has moved on to the controlled practice stage and the Ls are working in pairs. Weaker Ls are given a simple gapfill task to do with the missing words in a box, in jumbled order. Stronger Ls are given the same task, but without the examples in the box.
- the lesson has the aim of improving the Ls' ability to use the present continuous to express future arrangements. The class consists mainly of general purpose learners who are asked to explain their plans for the coming public holiday. However, there are also two Ls in the class who need English for Business Puposes. They are paired together and asked to tell each other about the plans for their next business trip - where they are going, why, who they are going with, where they are staying,, how long for etc.
For another example of how this might be done without needing to provide different materials or activities for the learners, see this article on the Test - Teach - Test lesson format. The article mentioned below by Rachael Roberts also addresses the same concerns.
Roberts, R. Simple ways to differentiate materials for mixed level classes
Swift, S. Teaching Mixed Ability Groups : A Solution
Dudley and Osvath, Mixed Ability Teaching, OUP
(Remember that Amazon often has much cheaper copies of the book you want, both new and used, than the ones advertised. It's always worth clicking on the link to check)