Functional Definition:

Ka is the dissociation constant and it is one way of telling you how strong an acid is. Ka is represented by the fractional relationship of product concentration over reactant concentration. Ideally if an acid is strong its products will have a higher concentration than its reactants making the Ka larger. If the acid is week the reactants will predominate and the Ka value will be low.

pKa is the -log of our Ka value. If you remember from your introductory chemistry course we also used a similar relationship for proton concentration called pH. For pKa we think of the relationship to acidity in reverse to that of Ka. If the Ka value is large then the pKa value will be small and our acid is strong. If the Ka value is small then our pKa is large and the acid is weak.