During my TEFL career, I was once part of a team employed to "convert" a school which used the Direct Method to one which would use the Communicative Approach. Why the difference in terms? What distinguishes a method from an approach?
Both methods and approaches are based on two sets of beliefs : a theory of language, and a theory of learning. So, for example, the Audiolingual Method was based on a theory of language emerging from structural linguistics, while its theory of learning was behaviourism. The Communicative Approach is based on a theory of language which emphasises not just form but also use (and which originated from the work of Wilkins, Van Ek and others on notional/functional syllabuses in the 1970s) while its learning theory draws on the precepts of a variety of sources including humanistic education and social constructivism.
So what's the difference? Basically, it lies in the degree of rigidity of the methodology and materials used. If a teacher is following a method, there will be a set sequence of steps for each lesson, which must be followed in order and in exactly the way set down by the Method. Some activity types will be prescribed, and others proscribed. For example, in the case of the Direct Method school I mentioned above, the teachers had a handbook which laid down the exact series of steps/activities (specified down to exactly what the teacher should say) to be followed in each lesson - and classes were regularly "inspected" to ensure that those steps were being followed. Even more recent methods will have a series of stages - those for Community Language Learning are described here - and a set of rules about what must/mustn't be done, when and how. CLL for example insists that the teacher must only answer questions asked by the learners, without volunteering information at any point, while the Audiolingual Method included the famous rule : Nothing should be spoken before it has been heard. Nothing should be read before it has been spoken. Nothing should be written before it has been read.
An approach, on the other hand, sets out the principles - the beliefs about language and learning - but leaves it up to the materials writer and/or the teacher to decide how those principles should be implemented in the classroom. There are no prescribed steps - although there may be commonly used lesson formats - and as long as they are compatible with the basic beliefs, any form of materials or methodological techniques might be used. So for example, presenting a decontextualised sentence like The cat will drink milk would not be acceptable in the Communicative Approach, as it does not adhere to the belief that use must be exemplified as well as form when teaching a language item. Instead, the model sentences would need to be contextualised and chosen so that their use was clear and could be explained to or inferred by the learners (eg in a dialogue between people planning a party and predicting what drinks they need to buy - The kids will drink Coca Cola).
As I suggested above, the Communicative Approach does not use a single theory of learning but draws on a number of educational approaches This means that it is eclectic in its methodology - ie the classroom techniques used are not prescribed and always the same - but may have their roots in a number of different theories. To take the example of our party planning dialogue, the lesson might start in the following format :
1. Pair work warm up : When did you last have a party or invite a number of friends to dinner? What did you decide to eat and drink? Why did you choose those items? This is a personalisation activity reflecting Humanism, Social constructivism, and Gardner's Multiple Intelligences (interpersonal intelligence)
2. Follow up to the discussion focusing what they talked about (Humanism) and on emergent language ("noticing", Swain's Output Hypothesis).
Presentation of the model dialogue based on comparison with their own discussion (Were their reasons for choosing the food and drink the same as yours or different?) Again, note the personalisation involved in asking them to relate the dialogue to their own discussion.
3. Pair work : A guided discovery activity focusing on the form and use of the target structure reflecting social constructivism,"noticing", humanism (the development of autonomy) and Gardner's Multiple Intelligences (logical-mathematical Intelligence).
4. Controlled practice - drilling of the model dialogue (behaviourism/Audiolingualism)
Etc. The lesson might continue with more controlled practice activities, semi-controlled and free practice. The only aspect of learning theory which could be considered specific to the Communicative Approach would be that by the end of the activity sequence, the learners should be given the chance to use the target language in a realistic communicative situation - eg an activity such as a roleplay or simulation, a discussion, or an information gap activity - which reflects normal conditions of communication : the text (written or spoken) is contextualised and the participants know the purpose of the communication, their relationships etc, and (if a spoken activity) it takes place spontaneously, so that they are listening and reacting to what their partners say, thinking of what to say and how to say it with real time.
And this is only one possible sequence - based very much on a theory of language learning rather than language acquisition and a PPP format. A teacher who took an acquisitional approach might develop a very different learning sequence - yet it would still relate to the communicative approach.
To see another example of how a lesson might be eclectic in format, see here - where I describe a sequence from a textbook teaching writing which combines both a genre and a process approach to teaching writing, while also being influenced by humanism and social constructivism.
Thornbury, S. 30 Language Teaching Methods, Cambridge