  We show how the photon statistics emitted by a large variety of light-matter
systems under weak coherent driving can be understood, to lowest order in the
driving, in the framework of an admixture of (or interference between) a
squeezed state and a coherent state, with the resulting state accounting for
all bunching and antibunching features. One can further identify two mechanisms
that produce resonances for the photon correlations: i) conventional statistics
describes cases that involve a particular quantum level or set of levels in the
excitation/emission processes with interferences occurring to all orders in the
photon numbers, while unconventional statistics describes cases where the
driving laser is far from resonance with any level and the interference occurs
for a particular number of photons only, yielding stronger correlations but
only for a definite number of photons. Such an understanding and classification
allows for a comprehensive and transparent description of the photon statistics
from a wide range of disparate systems, where optimum conditions for various
types of photon correlations can be found and realized.
