  We present the systematic analysis of the UVOT and XRT light curves for a
sample of 26 Swift Gamma-Ray Bursts (GRBs). By comparing the optical/UV and
X-ray light curves, we found that they are remarkably different during the
first 500s after the BAT trigger, while they become more similar during the
middle phase of the afterglow, i.e. between 2000s and 20000s. If we take
literally the average properties of the sample, we find that the mean temporal
indices observed in the optical/UV and X-rays after 500s are consistent with a
forward-shock scenario, under the assumptions that electrons are in the slow
cooling regime, the external medium is of constant density and the synchrotron
cooling frequency is situated between the optical/UV and X-ray observing bands.
While this scenario describes well the averaged observed properties, some
individual GRB afterglows require different or additional assumptions, such as
the presence of late energy injection. We show that a chromatic break (a break
in the X-ray light curve that is not seen in the optical) is present in the
afterglows of 3 GRBs and demonstrate evidence for chromatic breaks in a further
4 GRBs. The average properties of these breaks cannot be explained in terms of
the passage of the synchrotron cooling frequency through the observed bands,
nor a simple change in the external density. It is difficult to reconcile
chromatic breaks in terms of a single component outflow and instead, more
complex jet structure or additional emission components are required.
