Answer
The strength of a base measures its capacity of dissociating in water, and therefore, producing $[OH^-]$ and a conjugate acid $([B^+])$.
Bases that are capable of totally dissociating in water are called "strong bases".
But other bases, like $NH_3$, only dissociate in a certain value (not 100%), that depends on the concentrations, and can be calculated by the formula:
$Kb = \frac{[B^+][OH^-]}{[BOH]}$
Where each base has its $Kb$, that can be found in tables, books or the internet.
Work Step by Step
This information can be found in the section 15.6 (page 685) of this book.