In general acid catalysis all species capable of donating protons contribute to reaction rate acceleration. The strongest acids are most effective. Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example diazonium coupling reactions.
When keeping the pH at a constant level but changing the buffer concentration a change in rate signals a general acid catalysis. A constant rate is evidence for a specific acid catalyst.  When reactions are conducted in nonpolar media, this kind of catalysis is important because the acid is often not ionized.
Enzymes catalyze reactions using general-acid and general-base catalysis.