Having thrived on a diet of Umami, I now feel qualified in my attempt to describe this mysterious "fifth taste." Oh, and thanks Wikipedia!
Here goes. Umami's savory and hearty flavor basically comes from chemicals (amino acid) called glutamates. Glutamic acid pops up in many aged and fermented foods such as soy sauce, certain sharp and bitey cheeses, Vegemite (I'm talking to you, Sydney). As a sodium salt, monosodium glutamate, which we know as MSG, is used with a heavy hand in the food industry.
Also containing a high glutamate content are mushrooms, asparagus and tomatoes.
Umami, correct me if I'm wrong, roughly translates to good-eatin'