  In these lectures we start with a pedagogical introduction of the properties
of open and closed superstrings and then, using the open/closed string duality,
we construct the boundary state that provides the description of the maximally
supersymmetric Dp branes in terms of the perturbative string formalism. We then
use it for deriving the corresponding supergravity solution and the Born-Infeld
action and for studying the properties of the maximally supersymmetric gauge
theories living on their worldvolume. In the last section of these lectures we
extend these results to less supersymmetric and non-conformal gauge theories by
considering fractional branes of orbifolds and wrapped branes.
  Lectures given at the School "Frontiers in Number Theory, Physics and
Geometry", Les Houches, March 2003.
