  We present an analysis of early BAT and XRT data for 107 gamma--ray bursts
(GRBs) observed by the Swift satellite. We use these data to examine the
behaviour of the X-ray light curve and propose a classification scheme for GRBs
based on this behaviour. As found for previous smaller samples, the earliest
X-ray light curve can be well described by an exponential which relaxes into a
power law, often with flares superimposed. The later emission is well fit using
a similar functional form and we find that these two functions provide a good
description of the entire X-ray light curve. For the prompt emission, the
transition time between the exponential and the power law gives a well-defined
timescale, T_p, for the burst duration. We use T_p, the spectral index of the
prompt emission, beta_p, and the prompt power law decay index, alpha_p to
define four classes of burst: short, slow, fast and soft. Bursts with slowly
declining emission have spectral and temporal properties similar to the short
bursts despite having longer durations. Some of these GRBs may therefore arise
from similar progenitors including several types of binary system. Short bursts
tend to decline more gradually than longer duration bursts and hence emit a
significant fraction of their total energy at times greater than T_p. This may
be due to differences in the environment or the progenitor for long, fast
bursts.
