Classification of three-dimensional exceptional log canonical   hypersurface singularities I
  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. 
S.A. Kudryavtsev