We study statistical properties and a space-time structure of the systems formed in relativistic nuclear collisions. First, we find that the freeze-out conditions, like a temperature and velocity gradient near center of the fireball, are similar for different A+A collisions. Second, using these universal freeze-out parameters we analyze thermodynamical peculiarities of expanding fireballs and find the temperature of chemical freeze-out and chemical potentials including one associated with unique mean-field. The problem of the „pion deficit” for S+S CERN SPS is discussed also.