  Recent scenarios of phenomenologically realistic string compactifications
involve the existence of gauge sectors localized on D-branes at singular points
of Calabi-Yau threefolds. The spectrum and interactions in these gauge sectors
are determined by the local geometry of the singularity, and can be encoded in
quiver diagrams. We discuss the physical models arising for the simplest case
of orbifold singularities, and generalize to non-orbifold singularities and
orientifold singularities. Finally we show that relatively simple singularities
lead to gauge sectors surprisingly close to the standard model of elementary
particles.
