  After the first observations of W bosons in leptonic interactions, about 4000
WW candidate events per experiment have been collected at LEP2. This data
allows the measurement of the WW production cross section at different
centre-of-mass energies, as well as W decay branching fractions. The W hadronic
branching fraction can be converted into a test of the unitarity of the CKM
matrix, or into an indirect determination of the matrix element $|V_{cs}|$. A
more direct measurement coming from charm tagging is also performed. The W mass
has been measured via the cross section (in the threshold region) and the
direct reconstruction of the W decay products, using different techniques to
account for the distortions due to experimental effects. The main systematic
error to the mass reconstruction in the fully hadronic channel comes from QCD
effects like Color reconnections and Bose-Einstein correlations, extensively
studied in WW events. In $e^+e^-$ collisions W pairs can be produced in
s-channel via a three vector boson vertex, so a direct study of the trilinear
gauge boson couplings is possible. Modification of WW cross section and
distributions of W production and decay angles would be an indication of
non-standard couplings, thus a first hint for the presence of new physics.
