  This paper presents light curves and the first systematic characterization of
variability of the 106 objects in the Fermi Large Area Telescope (LAT) Bright
AGN Sample (LBAS). Weekly light curves obtained during the first 11 months of
survey (August 04, 2008 - July 04, 2009), are tested for variability, and their
properties are quantified through autocorrelation and structure function
analysis. For the brightest sources power density spectra (PDS) and fit of the
temporal structure of major flares is performed. More than 50% of the sources
are variable, where high states do not exceed 1/4 of the total observation
range. Variation amplitudes are larger for FSRQs and low/intermediate
synchrotron peaked (LSP/ISP) BL Lac objects. Autocorrelation time scales vary
from 4 to a dozen of weeks. Variable sources of the sample have 1/(f^{a}) PDS
and show two modes: (1) rather constant baseline with sporadic flaring activity
characterized by flatter PDS slopes resembling flickering and red-noise with
occasional intermittence, and (2) - measured for a few blazars showing strong
activity - complex and structured temporal profiles characterized by
longer-term memory and steeper PDS slopes typical of a random-walk underlying
mechanism. The average PDS slope of the brightest 22 FSRQs and the 6 brightest
BL Lacs is 1.5 and 1.7 respectively. The study of temporal profiles of well
resolved flares shows that they generally have symmetric profiles and that
their total duration vary between 10 and 100 days.
