Effective Conformal Descriptions of Black Hole Entropy: A Review
  Black holes behave as thermodynamic objects, and it is natural to ask for an underlying "statistical mechanical" explanation in terms of microscopic degrees of freedom. I summarize attempts to describe these degrees of freedom in terms of a dual two-dimensional conformal field theory, emphasizing the generality of the Cardy formula and the consequent universal nature of the conformal description. 
S. Carlip
gr-qc hep-th