The idea is that some people learn better when they hear speech, others hear better through music, others learn better from some sort of visual communication (whether graphs, pictures, video, classroom demonstrations, computer or tv graphics or video, etc.), others from acting out a performance or doing an experiment or playing with representative objects -- "manipulatives"-- or by writing, etc.).  Similarly, many students are thought to demonstrate their knowledge better if they can do it in some way other than writing down test answers.

Unfortunately, in regard to this latter point, many teachers misunderstand the concept and simply assign different "presentation (or communication) styles" to be used by everyone on alternating assignments instead of letting each child choose the manner s/he prefers.  The reason this defeats any possible benefit of the learning styles demonstration approach is that instead of helping everyone grade-wise, it hurts everyone.  Suppose for purposes of illustration that, say, one-third of the students do better on exams, one-third do better making posters or 3-dimensional projects, and one-third do better creating a group play or skit.  If on all the different assignments/tests each student could demonstrate their knowledge in the style they prefer, all students might earn high grades.  But if all the students must do one-third of their assignments in the first way,  one-third in the second way, and one-third in the last way, then quite conceivably all the students will do poorly on the two-thirds of the assignments in a demonstration style they are not good at.

But assuming there is not this misuse of the learning styles approach, still there are serious problems.  First, I agree that the more different ways a teacher can present material well, the more likely students will learn, and that the more ways a child can show the teacher he or she has learned something, the better. However, "learning styles" is not the Holy Grail of education that it is often touted to be because (1) teachers who cannot explain things well or who do not understand a concept well will not help anyone learn if they speak instead of write, or if they draw pictures instead of speaking, or if they design experiments or buy manipulatives, write plays or compose songs, and (2) certain concepts only lend themselves to certain sorts of presentations. If you want to learn about foods or wine, all the pictures and all the verbal  explanations, whether oral or written are not going to teach you what taste will. If you want to learn to ride a bicycle or throw a boomerang, at some point you are going to need to get on a bike or get a boomerang in hand, though you also may need some verbal or written instruction along with it.  But verbal, written, dramatic, and even demonstrational instruction is not likely to teach these things without some hands-on instruction and experience, regardless of whether a student's main learning style is verbal or not.

I had one college teacher who, when asked a question about something he had just said or that was assigned in the previous night's reading, would say it over again louder and louder and write it on the board in the same words, using different colors of chalk for each repetition, as if the reason we could not understand him was that we were either deaf or color blind.  It never occurred to him to try to explain something in different words or in a conceptually different way.  For him, using different colors and different vocal volumes was to explain it in a "different way".  The same problem occurs when one uses a different "style" but essentially conveys the same information, when it is the information conveyed that is problematic, not the manner in which it was conveyed. 

For example, mathematical "place value" in the primary grades is a difficult concept for most kids to understand. Most teachers have trouble teaching it to students because the notion of the same numbers' having different values depending on what column they are in does not make much sense to children, and because the idea of numbers being made up of different parts (hundreds, tens, ones) that are assembled together is complex and strange to them.  One teacher I know had purchased for her classroom a piece of demonstration equipment that had three vertical wires parallel to each other.  On each of the wires there were 10 balls, each with a different numeral on it.  There was a way to move the balls so that only three balls showed at a time, one on each wire.  So you could display the number, for instance, 646 on the device by moving the 6's into position on the hundreds wire and also on the one's wire, and by moving the 4 into position on the tens wire.  The teacher did not understand why the kids couldn't do this any better than they could write numbers on paper, using place value.  After all, this was kinesthetic and was supposed to appeal to kids who needed physical, concrete things to manipulate.  The problem was that it is the concept that is difficult, no matter whether it is represented two-dimensionally on paper or three-dimensionally.  There was nothing gained by putting numbers into some three-dimensional columns rather than two-dimensional ones.  It was like explaining something the same way in different colors.  It would be like trying to have a kid understand a vocabulary word he didn't know in a book by giving him a block of wood with the word carved on it.  An unknown word on a page isn't somehow more intelligible when carved on a wood block or even a whole tree trunk. 

There are a great many concepts and kinds of knowledge that are not amenable to being taught by use of different presentation styles or where style of information presentation is the least important factor; they require being taught well conceptually; they require the teacher's understanding how the concepts work and how the students are likely to misunderstand them and what specific kinds of explanations, demonstrations, leading questions, analogies, examples, pictures, manipulatives, metaphors, pointed humor, or information may help students comprehend.

For example, children who have trouble learning to ride a bicycle are not usually helped much by watching someone else ride one.  Nor are they helped much by being given a bicycle and being told to go to it and keep trying till they ride.  Nor are they helped usually by being given a bike with one training wheel removed. What they normally need is help understanding how to balance and a helping hand that gets them to "feel" their balancing point.  They need to understand that their hips and butt determine the balance of the bike more than does their shoulders, so that they need to shift their hips, not their shoulders, when they feel themselves starting to lean too far in one direction or another.  And they need to have their bicycle seat balanced and moved as they ride until they begin to feel how to move it themselves into what feels to them like a balanced position.

There are a number of papers on this site that give explanations of concepts or works (such as the Oedipus trilogy or Greek drama) that students often have difficulty understanding or appreciating.  Most of these papers exemplify ways to explain those things  -- ways which have nothing to do with learning "styles", and which a learning "styles" approach would not help any more than would writing out a bad explanation  in different colors or carving it on wood.  The idea behind the explanations is to demonstrate, by whatever means possible and/or necessary, the logic of the concepts involved in a way that a student is likely to "get it".  That is not always an easy thing to do; and it is not made automatic or easy by simply trying to do it in a style that appeals to a different sense or talent.