The physical properties of W UMa binary systems are revisited on the basis of the observational data published in the last decade and of the recent theoretical studies on angular-momentum-loss-driven secular evolution. The absolute elements (masses, radii, luminosities) are derived by an inference method and a calibration based on the available high quality spectroscopic orbits. The derived age (8Gy) agrees with the estimate of Guinan and Bradstreet from space motions. The analysis of the resulting physical parameters shows little correlation between the standard classification in A and W subtype (first proposed by Binnendijk (1970) and only related to the light curve morphology) and the evolutionary status and origin of the systems. Most A-subtype systems seem to have no evolutionary link with W-subtype ones. The relation between total mass and mass ratio for the "bona fide" sample also suggests that mass loss from the system may play an important role.