Cramér's V is a measure of association between two categorical variables.
Cramér's V is a measure of association. It is usually calculated by forming the contingency table and computing $\chi^2$. In the case of the $2 \times{} 2$ table it is the same as the $\phi$ coefficient and Cramér's V is sometimes referred to as Cramér's $\phi$.
The Wikipedia page has much more detail.
