The pKa value is used to express the strength of acids. It's defined as -log10 Ka where Ka is the acid dissociation constant.
An acid with low pKa value that completely ionizes in solution.
An acid with high pKa value that only partially ionizes in solution.
A solution containing both weak acid and its conjugate base (or vice versa) capable of resisting changes in pH upon addition of small amounts of either an acid or base.