Black holes are formed when stars with mass around a few times the mass of our Sun or higher collapse under their own gravitational field. Black holes are solutions of Einstein's equations which relate space-time to matter.

This thesis studies the semi-classical approach to quantization, treating gravity as classical while treating all other fields as quantum fields. This approach led to the discovery of the phenomenon of black hole radiation.

Ordinary thermodynamic systems have a statistical description in terms of microscopic constituents, like a gas having molecules as its microscopic constituents. This thesis precisely addresses the question: Does a black hole have such a description?

String theory describes all interactions, including gravity, in a unified framework and provides a complete theory of the quantum nature of space-time. This thesis studies how string theory helps in giving a microscopic description of black hole thermodynamics.

The thesis reviews aspects of black hole thermodynamics and semi-classical gravity, including Hawking radiation. It also explains D-branes, which are some of the solitonic objects that describe black holes.

Additionally, this thesis describes anti-de Sitter spaces which help to understand the properties of certain black holes with the help of a recent conjecture. This conjecture relates physical quantities on anti-de Sitter spaces to those of a conformal field theory living on its boundary.

For certain black holes, like the Schwarzschild black hole, the Greybody factor is frequency-independent and equals the area of the horizon for low frequencies. The spectrum for these black holes is very similar to black body radiation. For other black holes, this factor plays a crucial role.

The researcher studies the back reaction of scalar and fermion matter fields on the black hole geometry. These fields have support on space-like slices very near the horizon. The interactions of the fermionic and scalar outgoing and infalling fields for the Schwarzschild black hole are explored, showing their importance in deriving Hawking radiation.

The extremal black hole entropy is considered, and it is observed that semiclassical derivations suggest it to be zero. This thesis examines the case of extremal black holes in String theory and proposes a resolution for the apparent contradiction.

Hawking radiation from a 4-dimensional black hole obtained by compactifying M theory on T^7 is discussed. The radiation rate has a structure reproducible from a 1+1 conformal field theory. A microscopic description of fermionic radiation from the D1-D5 black hole is provided.

The 2+1 dimensional BTZ black hole and its associated fermionic radiation are discussed. The radiation from a 5-dimensional black hole, whose near horizon geometry is a product of the 2+1 dimensional BTZ black hole and a compact manifold, is also covered.

The emission rates for scalars, fermions, and vectors are determined by probing the near horizon geometry. The thesis concludes with a discussion on the current understanding of black hole thermodynamics.