We review the recent developments in superstrings. We start with a brief
summary of various consistent superstring theories and discuss T-duality which
necessarily leads to the presence of D-branes. The properties of D-branes are
summarized and we discuss how these suggest the existence of 11-dimensional
quantum theory, M-theory, which is believed to give rise to various
superstrings as perturbative expansions around particular backgrounds in the
theory. We also discuss the interpretation of brane solutions as black holes in
string theories and statistical explanation of Bekenstein-Hawking entropy. The
idea behind this interpretation is that there is a fundamental duality between
closed (gravity) and open (gauge theory) string degrees of freedom, one of
whose manifestation is what is kown as AdS/CFT correspondence. The idea is used
to discuss the greybody factors for BTZ black holes. Finally the entropy of
various balck holes are discussed in connection with Cardy-Verlinde formula.Comment: 28 pages, 7 figures, Latex. Lectures at the international workshop
``Brane world'' at YITP, 15--18 January 2002, to be published in Prog. Theor.
Phys. Suppl. ptp style files included. v2: minor corrections, v3: minor
corrections and refs. added, v4: eq.(7.14) correcte