String theory is a remarkable achievement in theoretical high energy physics which aims to unify the fundamental particle interactions under a single framework. My fascination in it stems from its ability to provide a framework for addressing issues related to Quantum Gravity and to provide fascinating insights into QFTs through various dualities.

String theory at a basic level deviates from quantum field theory by positing that at extremely small length scales the particles seen in nature are actually one-dimensional and quantizes these strings to obtain an understanding of the fundamental forces. This picture has been significantly modified in the last decade and a half with the discovery of other interesting objects in string theory, viz., D-branes. D-branes are extended objects on which open strings can end, and have interesting dynamics of their own. Further, they offer a non-perturbative window into string theory and have been central players in modern developments of the subject.

References:

A good popular level introduction to string theory can be found in

• The Elegant Universe by Brain Greene.

An extremely well written text-book which should be accessible to undergraduates having done Quantum Mechanics is:

• A First Course in String Theory by Barton Zwiebach.

There are of course other text-books on the subject including the classics:

• Superstring theory by Michael Green, John Schwartz and Edward Witten.
• String Theory (Vols. 1 & 2) by Joe Polchinski

More recent expositions of the subject are:

• String theory and M-theory by Kartin Becker, Melanie Becker and John Schwartz.
• String Theory in a nutshell by Elias Kiritsis

Other interesting reading available on the e-print archive:

• Emergent spacetime by Nathan Seiberg: hep-th/0601234
• Spacetime in string theory by Gary Horowitz: gr-qc/0410049