What is "Teichmüller Theory" and its history? What is "Teichmüller Theory"? What part has been worked out / foreseen  by O. Teichmüller himself and what is further development? Is there some current work which might be considered as continuation/completion of this theory? 
Background The question might be seen as too naive and can be answered by google  or Wikipedia, but I have it in mind for a long time and do not think  that it is that much simple. Let me explain what is puzzling me: Teichmüller space is very close to moduli space of Riemann surfaces ("The Teichmüller space is the universal covering orbifold of the (Riemann) moduli space.") and reading some sources make me expression that  "Teichmüller Theory" is  everything which is related to the moduli space of Riemann surfaces. Is it really like this ? If it is true it does not seem to me good name since "theory" should be something not so diverse as current research on moduli spaces of Riemann surfaces.
On the other hand what I heard about the contribution of Teichmüller himself - it is introduction of the Teichmüller metric by means of quasiconformal maps. (See Wikipedia).
It is beautiful result, but it is kind of "theorem", not "theory", so probably there is something more? which I am missing?
The question might be considered as background to  What are some Applications of Teichmüller Theory? 
 A: First of all, let me recommend a book: J. Hubbard, Teichmüller theory, vol. 1.
Let me try to list briefly Teichmüller's own contribution to Teichmüller theory.
Bers's papers of 1960-s are good primary sources. The few papers of Teichmüller himself
that I read are also exciting, but my poor knowledge of German does not allow me to
read all of them. 
Perhaps the main contribution is the introduction of Teichmüller's space
(instead of the much more complicated moduli space). It is simply connected!
The second main contribution is the definition of the Teichmüller metric on this space.
The metric is defined using
a solution of an extremal problem: finding a quasiconformal homeomorphism in a
homotopy class with smallest dilatation. Such problems in plane domains were first
considered by Grötzsch. Teichmüller's contribution was
a) considering them on compact Riemann surfaces, and
b) describing the extremal map in terms of a certain quadratic differential.
He also established existence and uniqueness of the extremal mapping with a very
original argument.
Teichmüller distance is defined as $(1/2)\log K$, where $K$ is the extremal
dilatation. 
Teichmüller died young (he was killed or MIA in the Eastern front, somewhere near Kiev
in 1943), and many of his principal papers contain a lot of heuristic arguments.
The subject was developed by Ahlfors and Bers in 1950-s. They rigorously introduced
the analytic structure on Teichmüller spaces, and proved in particular that the Teichmuller
space of surfaces of genus $g>1$ is isomorphic to a domain in $C^{3g-3}$ which is
homeomorphic to $R^{6g-6}$.
They identified the cotangent space as a space of quadratic differentials. Later Royden proved
that the Teichmüller distance coincides with the Kobayashi distance.
The crucial technical tool, existence and analytic dependence on parameters of 
the homeomorphic solution of the Beltrami equation with $L^\infty$ norm of the coefficient less
than 1,
which people call sometimes the "Measurable Riemann theorem", was not available in
Teichmüller's time. It was published for the first time by Boyarski in 1955.
This more or less constitutes the original "Teichmüller theory".
Later the meaning of the term substantially expanded, to include almost everything
about the moduli spaces. 
EDIT. The good news is that all principal papers of Teichmüller are now available in English:
MR3560242
Handbook of Teichmüller theory. Vols. IV,V,VI. 
European Mathematical Society (EMS), Zürich, 2016. (Each volume contains translations of several papers of Teichmüller)
And with comments.
