Cramer's V for a r x c table is defined as
sqrt [ (chi-square) / (N Min(c-1,r-1)) ]
It converts a chisquare from a two-way frequency table with a set number of degrees of freedom into an effect size. Cohen (1988) gives rules of thumb for Cramer's V using chi-squares obtained from frequency tables having 1, 2 and 3 degrees of freedom (df) with a total sample of size N (see here which is taken from here).
In the case of a 2x2 table Cramer's V is equivalent to the Phi statistic.