In deriving pKa we consider how much hydronium ion is formed when an acid is dissolved in water. So in deriving pKb we consider how much hydroxide ion a base will form in solution.
Like Ka though, we usually express Kb as a logarithm.
pKb = -log Kb
Therefore the scale is the same as the acid scale where the lower the pKb the stronger the base
The pKa scale however is more widely used and as the two scales are related one scale can suffice, therefore many textbooks and information sources will often only use pKa values.
The next page discusses factors which affect the basicity of a base.