The first time I seriously turned my attention to umami was last summer at Copia, during a wine tasting (a rather blotto wine tasting, for me). In addition to sweet and sour, bitter and salty, they introduced umami as the fifth taste sensation to hit our taste buds. Luckily, we kinda skipped right over it, because after my 5th glass of wine, I wouldn't have known umami from my real mommy. The Wall Street Journal has an interesting article about it in their weekend section. Luckily this is one of the few pieces that they make available online to the riffraff, otherwise known as non-subscribers.
The essence of umami is that savory, full mouth feel you get from certain foods. It is found naturally in meat, fish, some vegetables and dairy products. Two of the best examples of umami in the West are Parmesan cheese and shitake mushrooms. When umami is stripped down to its most basic chemical component, it is composed of glutamate, but don't mistake it as salty. Glutamate, of course, is the major ingredient in monosodium glutamate, the bad boy of additives.
But glutamate is not the sodium-laden, headache-producing element you might think. There are specific receptors on the tongue for it, just as there are for the other 4 tastes. While it is difficult to detect umami elements all by themselves, they give a dish a more complete full flavor than if they were left out.
Interestingly, every region of the world has its own unique umami condiments, which improve mouth feel: tomato ketchup from the U.S., Worcestershire sauce from Britain, soy sauce from Asia and anchovy paste from Europe.
Here is Gary Danko demonstrating a perfect umami dish, Roasted Tomato Soup (from the WSJ website):