This article is from the The Simpsons FAQ, by Ondre Lombard email@example.com and Andrew Gill firstname.lastname@example.org with numerous contributions by others.
"Springfield" is a fictional location. MG says he chose the name because
it is one of the most common city names (23 Springfields exist in the U.S.)
and the setting of the antithesis to the Simpsons, "Father Knows Best".
Indeed, from Groening's childhood perspective, that Springfield was "the
next town over" from his home in Portland, Oregon.
According to Producer/Director David Silverman, Springfield is in the state
of North Takoma, eight miles from Toon Town (although it appears this still
isn't to be considered an "official" answer.) In Mr Lisa Goes to
Washington, the Simpsons' address is 59 Evergreen Terrace, Springfield,
TA--TA probably representing Takoma. See next question.
The bottom line, though, is that even when the series makes reference to
places and events unique to a specific Springfield, it does not mean that
the series takes place in that given state.
According to some sources on the group, Matt Groening has said that
although Springfield is basically "anytown USA" it does have features
somewhat similar to towns in Oregon, where Matt grew up.
-> Read "Where Is Springfield" if you're still not convinced. (See Sec 3.2)