  Observations of very high energy gamma-rays from blazars provide information
about acceleration mechanisms occurring in their innermost regions. Studies of
variability in these objects allow a better understanding of the mechanisms at
play. To investigate the spectral and temporal variability of VHE (>100 GeV)
gamma-rays of the well-known high-frequency-peaked BL Lac object PKS 2155-304
with the H.E.S.S. imaging atmospheric Cherenkov telescopes over a wide range of
flux states. Data collected from 2005 to 2007 are analyzed. Spectra are derived
on time scales ranging from 3 years to 4 minutes. Light curve variability is
studied through doubling timescales and structure functions, and is compared
with red noise process simulations. The source is found to be in a low state
from 2005 to 2007, except for a set of exceptional flares which occurred in
July 2006. The quiescent state of the source is characterized by an associated
mean flux level of 4.32 +/-0.09 x 10^-11 cm^-2 s^-1 above 200 GeV, or
approximately 15% of the Crab Nebula, and a power law photon index of 3.53
+/-0.06. During the flares of July 2006, doubling timescales of ~2 min are
found. The spectral index variation is examined over two orders of magnitude in
flux, yielding different behaviour at low and high fluxes,which is a new
phenomenon in VHE gamma-ray emitting blazars. The variability amplitude
characterized by the fractional r.m.s. is strongly energy-dependent and is
proportional to E^(0.19 +/- 0.01). The light curve r.m.s. correlates with the
flux. This is the signature of a multiplicative process which can be accounted
for as a red noise with a Fourier index of ~2. This unique data set shows
evidence for a low level gamma-ray emission state from PKS 2155-304, which
possibly has a different origin than the outbursts. The discovery of the light
curve lognormal behaviour might be an indicator ..
