This video from Seeker gets down to the nitty-gritty of how we taste things, and how we sense flavors that are sour. While we know a lot about how it happens, we are still not sure exactly why sour flavors are a little different from the others.
Taste receptors are located near the bumps on your tongue, called the papillae. Gustatory cells reside there, and those cells take in the perception of taste and transmit it to your brain.
While you may have once heard that different tastes are sensed on different parts of the tongue, that actually isn't the case. Some areas of the tongue appear to be a bit more sensitive to certain flavors, but all of them can be tasted anywhere on the tongue.