  In this thesis we discuss some nonperturbative and noncommutative aspects of
string theory. We present low-energy background field solutions corresponding
to various D-branes (and their bound states) and intersecting branes in flat
and pp-wave spacetime. A class of D-brane bound states are constructed from
charged macroscopic strings and they are shown to satisfy the mass-charge
relationship of 1/2 BPS bound states. Another class of D-branes and
intersecting branes are constructed, by solving the type II field equations
explicitly, in pp-wave background with constant and non-constant flux. The
worldsheet construction of these D-branes and their spacetime supersymmetry
properties have also been analyzed in some detail.
  Further, we study an example of open strings ending on D-branes with mixed
boundary condition. In this context, we analyze various open and mixed sector
tree-level amplitudes of N=2 strings in the presence of constant NS-NS
antisymmetric tensor (B) field.
