  We present Period-Luminosity-Color and Period-Luminosity relations of
classical Cepheids constructed for about 1280 Cepheids from the LMC and 2140
from the SMC. High quality BVI observations (120-360 epochs in the I-band and
15-40 in the BV-bands) were collected during the OGLE-II microlensing
experiment. The I-band diagrams of the LMC show very small scatter,
\sigma=0.074 mag, indicating that Cepheid variables can potentially be a very
good standard candle. We compare relations of fundamental mode Cepheids from
the LMC and SMC and we do not find significant differences of slopes of the
Period-Luminosity- Color and Period-Luminosity relations in these galaxies. For
the first overtone Cepheids a small change of the slope of Period-Luminosity
relation is possible. We determine the difference of distance moduli between
the SMC and LMC with Cepheid relations and compare the result with difference
obtained with other standard candles: RR Lyr and red clump stars. Results are
very consistent and indicate that the values of zero points of the fundamental
mode Cepheid relations are similar in these galaxies. The mean difference of
distance moduli between the SMC and LMC is equal to \mu_SMC-\mu_LMC=0.51\pm0.03
mag. We calibrate the Period-Luminosity-Color and Period-Luminosity relations
for classical, fundamental mode Cepheids using the observed LMC relations and
adopting the short LMC distance modulus, \mu_LMC=18.22\pm0.05 mag, resulting
from the recent determination with eclipsing system HV2274, RR Lyr and red
clump stars and consistent with observations of Cepheids in NGC4258 galaxy, to
which precise geometric distance is known. Finally, we determine a constraint
on the absolute magnitude of Cepheids by comparison of their mean V-band
magnitude with that of RR Lyr stars in both Magellanic Clouds.
