  In this paper, which is a revised version of the author's PhD thesis, we
analyze two different applications of string theory. In the first part, we
focus on four dimensional compactifications of Type II string theories
preserving N=1 supersymmetry, in presence of intersecting or magnetized
D-branes. We show, through world-sheet methods, how the insertion of closed
string background fluxes may modify the effective interactions on Dirichlet and
Euclidean branes. In particular, we compute flux-induced fermionic masses. The
generality of our results is exploited to determine the soft terms of the
action on the instanton moduli space. Finally, we investigate how fluxes create
new non-perturbative superpotential terms in presence of gauge and stringy
instantons in SQCD-like models. The second part is devoted to the description
of statistical systems through effective string models. In particular, we focus
our attention on (d-1)-dimensional interfaces, present in particular
statistical systems defined on compact d-dimensional spaces. We compute their
exact partition function by resorting to standard covariant quantization of the
Nambu-Goto theory, and we compare it with Monte Carlo data. Then, we propose an
effective model to describe interfaces in 2d space and test it against the
dimensional reduction of the Nambu-Goto description of the 2d interface.
