Umami is gaining acceptance in the west as the fifth 'taste' (in addition to the traditional bitter, sweet, salty and sour), but its existence has been recognised in Asia, especially Japan, for about a century.
What is it? It's often described as 'savouriness' or 'deliciousness' - a rich meaty roundness that fills the mouth. It was identified by Japanese chemistry professor Kikunae Ikeda, who noticed that kombu (dried giant kelp, a staple in the Japanese kitchen) enhanced the flavour of whatever savoury foods it was cooked with (umami is not found in sweet dishes). He isolated the flavour-enhancing essence (which he named umami) and discovered that it was glutamate, the sodium salt of glutamic acid, an amino acid that's found in all animals and some plants.
How is it available? Umami is found naturally in varying degrees in meats, many vegetables (such as tomatoes, peas and mushrooms), milk (including from humans), cheese and seafood. In Chinese cuisine, you'll find umami in staple ingredients such as dried mushrooms, wind-dried meats (such as sausage, bacon and ham), soy and oyster sauces, dried seafoods and preserved vegetables. Umami is also extracted through a fermentation process from plant products such as sugar beet, tapioca, wheat, corn and soya beans - in which case it takes the shape of small white crystals and is known as monosodium glutamate (MSG).
What else? While nobody complains about side effects from natural forms of umami, there is some controversy about MSG - usually when it's overused at restaurants. It's been examined carefully in clinical double-blind studies but the results are inconclusive.
How to use? It can be as simple as finishing a pasta or noodle dish by sprinkling it with parmesan cheese or dried shrimp roe, or combining ingredients so the umami-effect is multiplied (think of Chinese superior stock that combines chicken and pork with dried scallops and ham, or the basic Japanese dashi broth, which uses katsuo [dried smoked bonito] and kombu to give a super-strong dose of umami).
MSG can be found in both the expected (bouillon cubes) and unexpected (some toothpaste and soft drinks). Used sparingly, MSG can deepen the flavour without the need to add a lot of salt.