Give a description of M-theory your grandmother can understand Inspired by this question, let me ask a similar question. Is it possible to do the same (give a description of M-theory your grandmother could understand)for M theory? While I know even experts don't understand it properly, I still hope some basic ideas may be expressed. Is it just a hypothetical non-perturbative formulation of quantume gravity whose different limits for varisous coupling constants are five string theories and 11d supergravity or theorists know about it in more detail?
Edit:
As far as my grandmother is concerned let me inform that she has a Phd (HEP her specialization) and quite a smart woman. She has also a good knowledge of GR. So she won't mind technical staff. What she finds frustrating is that there is no rigorous definition of M-theory available to her. In her age (80) she doesn't hope to understand every details of M-theory but she certainly can find some definition of M-theory helpful.
 A: Here is something, which may be aiming a little low...
The main two ways we describe our universe, quantum mechanics and general relativity, contradict each other when applied simultaneously. This seems to point out that the quantum nature of spacetime itself needs to be understood better. 
One way to resolve theoretical problems with our current understanding of spacetime is to embed it in a larger theoretical structure, which has powerful underlying symmetries. Those symmetries cannot be too restrictive: they should be enough not only to make the model well-behaved, but also to be consistent with the not-quite-so-symmetric world we see around us. 
This way of thinking has led to the theoretical structure of string theory and then M-theory. To study the structure of the theory, it is useful to first concentrate on the most symmetric situations, even though these are the most removed from our world. At first, this led to study of supersymmetric string theories in 10 dimensions (higher dimensional theories are more symmetric - their Lorentz invariance is larger and more restrictive). Later it turned out that those are all secretly related to an even larger  and more symmetric structure, dubbed M-theory, which describes all the previously known string theories as well as 11-dimensional supergravity.
The story is not finished, we only have bits and pieces of the underlying structure that is M-theory. But, we do have many indications we are on the right track. As always with deep structures we found side benefits in the form of unexpected applications in mathematics and physics. 
One of the unexpected discoveries is that quantum gravity is not all that different from other parts of physics, and sometimes conventional physics can be reformulated in different variables to make it equivalent to a quantum gravitational theory. Using classical and semi-classical gravity calculations then helps us explore conventional physics in regimes otherwise inaccessible. This is the whole subject of holography and its applications.
So, what we seem to have found is, instead of a specific model to describe our universe at short distances (or high energies), a whole new language in which we describe and discuss physics - and not just high energy physics. Where precisely this is going to lead is anyone’s guess.
Now, if your grandmother feels patronized, has more knowledge of physics and would like to ask more specific questions, I can try to add more details. It is a very large subject...
(see also the answers to this similar question)
A: I tend to explain this with analogues with instruments.  I say imagine a string, like a guitar string or a piano string.  However, this string has some interesting properties.  The tension in the string is due to the quantum uncertainty principle.  Because quantum mechanics says we are not able to identify the position of the string to complete precision while also knowing its momentum, this means there is a constant wavy motion which occurs.  This waviness happens whether we are pulling on the string or not.  So the string has its own tension.
Now for the open string there are these flapping open ends, a bit like bullwhips cracking in space.  This means that waves are reflected off the ends and waves going up and down are the same.  This is different from a closed string, which is a loop that have waves going clockwise and counter-clockwise (left & right) which are independent.  Now let us suppose we take that open string and put in on a sound board. The end points are now held fixed and a wave which approaches the endpoint is reflected off by this fixed point.  However, the sound board, which we will now call a membrane, or D-brane, may adjust in response.  So that reflected wave on the string may be changed in a way so that it has an independent part from the incoming wave.  This means the wave dynamics on that open string tied to D-brane becomes similar to the wave dynamics on the closed string.
In this way the behavior of quantum strings with respect to these D-branes, which are similar to Fermi surfaces or “defects” in space (with a follow on discussion on Fermi surfaces etc) can be transformed from one type to the other.  
I mean really, if you are talking to “grandmas” discussions about symmetries and the like will be little more than soporific.  Gotta keep it really simple, even if maybe slightly wrong. In the case of sb1's gramma this could be a sort of spring board for going into deeper discussions.  However, this gives at least a physical sense of M-theory.
A: M-theory is a conjectured 11-dimensional theory with the property that, compactified on a circle, it becomes Type IIA string theory, and, compactified on a line segment, it becomes E8 x E8 heterotic string theory. At some level of description, it should consist of 2- and 5-dimensional supermembranes coupled to the fields of 11-dimensional supergravity. An exact formulation is not known, but arXiV:1012.0459 reviews recent progress. 
