Even though we were taught in school that there are four flavors (sweet, sour, salty, and bitter)there is now a fifth: umami. Well, actually, there have been five known since 1908, but no one told elementary school teachers that, in part because scientists didn't isolate the taste buds that match it until 2000. MSG is the pure version of the flavor, like sugar is for sweet; umami can also be tasted in steak, mushrooms, anchovies, ketchup, and parmesan, but I can't figure out exactly what the connections are - I can't isolate it as a separate flavor. According the the WSJ article, it is "usually defined as a meaty, savory, satisfying taste". And it is popular right now because umami can be used to decrease the salt needed to make a meal taste good.
To learn more about umami, see this nice collection of articles on Finding Dulcinea. The linked article from the Wall Street Journal is especially helpful.