  All varieties, extremal contractions, singularities are divided on
exceptional and non-exceptional ones.
  Roughly speaking, there are the infinite families of non-exceptional
varieties, extremal contractions or singularities and only the finite number of
the types of exceptional ones.
  This subdivision is well demonstrated by the example of Du Val singularities.
There are two infinite series of non-exceptional singularities: $A_n$ and $D_n$
and only three types of exceptional ones: $E_6$, $E_7$ and $E_8$.
  Also the importance of exceptionality phenomenon follows from the next
observation: A). If a variety, extremal contraction or singularity is
non-exceptional then the linear system $|-nK_X|$ must have a "good" member for
small $n$. For example we can take $n\in \{1,2\}$ for the two-dimensional
singularities and $n\in \{1,2,3,4,6\}$ for the three-dimensional singularities.
B). Exceptional ones are "bounded" and can be classified.
  Using the inductive method of algebraic variety classification it was
obtained the description of three-dimensional exceptional hypersurface
singularities in this paper.
