  We provide a unified treatment of classical and quantum Gaussian-state
sources that unambiguously identifies which features of ghost imaging are
strictly quantum mechanical. We show that ghost-image formation is
fundamentally classical, with the image being expressible in terms of the
phase-insensitive and phase-sensitive cross correlations between the detected
fields. We then consider ghost-imaging scenarios with either phase-insensitive
or phase-sensitive sources, where the former are always classical but the
latter may be classical or quantum mechanical. We show that if their
auto-correlations are identical, then a quantum source provides resolution
improvement in its near-field and field-of-view improvement in its far field
when compared to a classical source.
