  We present a tight binding description of electronic properties of the
interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO). The description assumes
LAO and STO perovskites as sets of atomic layers in the $x$-$y$ plane, which
are weakly coupled by an interlayer hopping term along the $z$ axis. The
interface is described by an additional potential, $U_0$, which simulates a
planar defect. Physically, the interfacial potential can result from either a
mechanical stress at the interface or other structural imperfections. We show
that depending on the potential strength, charge carriers (electrons or holes)
may form an energy band which is localized at the interface and is within the
band gaps of the constituting materials (LAO and STO). Moreover, our
description predicts a {\it valve effect} at a certain critical potential
strength, $U_{0cr}$, when the interface potential works as a valve suppressing
the interfacial conductivity. In other words, the interfacial electrons become
dispersionless at $U_0= U_{0cr}$, and thus cannot propagate. This critical
value separates the {\it quasielectron} ($U_0<$ $U_{0cr}$) and {\it quasihole}
($U_0>$ $U_{0cr}$) regimes of the interfacial conductivity.
