  We present UBVI photometry for about 60,000 stars in a region of 44'x33'
centered on the Galactic globular cluster (GGC) NGC 2808. The central region
has been mapped with the HST WFPC2 camera. We cover a field which extends from
the center to about 1.7 tidal radii. We confirm the anomalously elongated HB
with gaps along it, and we show that the gaps are statistically significant. We
find that: 1) the extended blue tail of HB (EBT) is present beyond
400''(=9r_h), and extends to V=21.2 also in these external regions; 2) also the
gaps on the EBT are present at least out to 400'' from the cluster center, and
possibly beyond that. The location of the gaps in the HB seems to be the same
all over the cluster. 3) there are no significant radial gradients in the
distribution of the stars in the HB. The observational facts presented in this
paper seem to exclude the possibility that the EBTs originate from tidal
stripping of the envelope of the RGB stars due to close encounters in high
density environments or to mass transfer in close binaries. Also, it is not
clear whether the presence of EBTs and gaps on them are the manifestation of
the same physical phenomenon. We have also shown that the jump recently
identified in the HB using Str"omgren-u photometry is clearly visible also in
the U, U-B plane, at T_e~11,600K. Finally, we present the luminosity function
(LF) of the RGB. The LF clearly shows the presence of the usual RGB bump, but
also a second feature, 1.4 magnitudes brighter in V, that we have named RGB
heap. The RGB heap, visible also in other GGCs, is a new feature that, because
of its position on the RGB, we have tentatively associated with the recently
discovered K giant variables.
