What is the difference between pH and pKa?

pH is the measure of the concentration of proton in an aqueous environment, which is denoted as: pH = -log[H₃O⁺]. pKa however, is prefered when comparing how strong an acid is because it's the pH at which the concentration of the acid and the conjugate base is equal in concentration, which is denoted as: pKa = -log₁₀K_a where K_a is the acid dissociation constant: K_a = [H₃O⁺][A^-] / [HA].