An accessible and pedagogical introduction to the subject, this book provides a detailed and self-contained exposition of the main concepts of string theory. The first part deals with the basic concepts, reviewing special relativity and electromagnetism while introducing the concept of extra dimensions. D-branes and the classical dynamics of relativistic strings are discussed next, and the quantization of open and closed bosonic strings in the light-cone gauge, along with a brief introduction to superstrings. The second part begins with a detailed study of D-branes followed by string thermodynamics. It discusses possible physical applications, and covers T-duality of open and closed strings, electromagnetic fields on D-branes, Born-Infeld electrodynamics, covariant string quantization and string interactions. Primarily aimed as a textbook for advanced undergraduate and beginning graduate courses, it will also be ideal for a wide range of scientists and mathematicians who are curious about string theory.