pH is the negative of the (base 10) logarithm of the activity of H^+^: pH = -log10(aH+)

If you mean "what's the difference between concentration and activity," activity is the "effective concentration" of a species. For ideal solutions, activity is equal to concentration. For real solutions, interactions between the components in the solution may cause a species to "act" like it is more or less concentrated.

Dilute solutions at standard conditions are close to ideal: activity is about equal to concentration. But consider a concentrated solution of a salt: the activity will tend to be lower than the concentration because the cations and anions are not completely independent as in an ideal solution, but tend to "shield" each other due to electrostatic forces.