Well, the whole thing is very complicated. The asorbic acid in sour, compliments the falic acid in other sour foods. It may seem more sour to you if you taste sour on sour, but it is just your taste...
Usually, citric acid. Look it up, there's tons of info. For the extreme-pucker-up candies, there's malic acid, which is so strong that dentists use it to dissolve peoples' enamel before tooth surgery.