Global properties of X-ray afterglows of GRB

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.