  In this paper we review the basic Poissonian formulation of quasar
variability, using it as a mathematical tool to extract relevant parameters
such as the energy, rate and lifetimes of the flares through the analysis of
observed light curves. It is shown that in this very general framework the well
established anti-correlation between variability amplitude and wavelength can
only be understood as an effect of an underlying spectral component which
remains stable on long time-scales, and is redder than the variable component.
The formalism is applied to the B and R light curves of 42 PG quasars collected
by the Wise Observatory group. Variability indices for these data are obtained
with a Structure Function analysis. The mean number of living flares is
constrained to be in the range between 5 and 100, while their rates are found
to be of order 1--100 per yr. Monochromatic optical flare energies of
10^{46-48} erg/A and life-times of 0.5 to 3 yr are derived. Lower limits of
typically 25% are established for the contribution of a non-variable component
in the R band. The substantial diversity in these properties among quasars
invalidates simple versions of the Poissonian model in which flare energies,
lifetimes and the background contribution are treated as universal invariants.
The good correlation between the EW(H_beta) and the long term variability
amplitude is interpreted in a scenario where only the variable component
participates in the ionization of the line emitting gas. This idea is
consistent with the observed trends of the variability amplitude with lambda,
EW(HeII) and the X-ray to optical spectral index (abridged).
