  We perform time-resolved spectroscopy on the prompt emission in gamma-ray
bursts (GRBs) and identify a thermal, photospheric component peaking at a
temperature of a few hundreds keV. This peak does not necessarily coincide with
the broad band (keV-GeV) power peak. We show that this thermal component
exhibits a characteristic temporal behavior. We study a sample of 56 long
bursts, all strong enough to allow time-resolved spectroscopy. We analyze the
evolution of both the temperature and flux of the thermal component in 49
individual time-resolved pulses, for which the temporal coverage is sufficient,
and find that the temperature is nearly constant during the first few seconds,
after which it decays as a power law with a sample-averaged index of -0.68. The
thermal flux first rises with an averaged power-law index of 0.63 after which
it decays with an averaged index of -2. The break times are the same to within
errors. We find that the ratio of the observed to the emergent thermal flux
typically exhibits a monotoneous power-law increase during the entire pulse as
well as during complex bursts. Thermal photons carry a significant fraction
($\sim$ 30 % to more than 50%) of the prompt emission energy (in the observed
25-1900 keV energy band), thereby significantly contributing to the high
radiative efficiency. Finally, we show here that the thermal emission can be
used to study the properties of the photosphere, hence the physical parameters
of the GRB fireball.
