In this paper we review the general properties of X-ray afterglows. We
discuss in particular on the powerful diagnostics provided by X-ray afterglows
in constraining the environment and fireball in normal GRB, and the
implications on the origin of dark GRB and XRF. We also discuss on the observed
properties of the transition from the prompt to the afterglow phase, and
present a case study for a late X-ray outburst interpreted as the onset of the
afterglow stage.Comment: 8 pages, 1 color figure. Accepted for publication in "il nuovo
cimento". Proceeding of the 4th Rome GRB conference, eds. L. Piro, L. Amati,
S. Covino, B. Gendre. Corrected a typo in caption of Fig.
