http://en.wikipedia.org/wiki/Soccer_mom
The term is thought to have been coined by a woman running for city council in Denver, Colorado. When asked for her credentials, she replied: "soccer mom." It came into widespread use in the 1990s, notably during the 1992 and 1996 presidential campaigns and since.
Literally, soccer moms drive their children to play soccer. Soccer is considered a safer and more genteel competitive sport than American football, which is more prominent than soccer in mainstream American culture. At once, the soccer mom is associated with encouraging, if perhaps overscheduling, their children with activities, and with making personal sacrifices for their children's benefit (taking them to play), while perhaps remaining somewhat overprotective (by taking them to play soccer rather than, say, American football).
The term has found a life as shorthand for a stereotype, or set of stereotypes, far beyond its literal meaning.
Most widely, perhaps, they are associated with driving Volvos, minivans, or SUVs. Active pride in their children may be displayed, for instance, with membership in a Parent-Teacher Association, or with a bumper sticker boasting that her child is an honors student at their school.
The term was used as the basis for the song "Volvo-Driving Soccer Mom" by the band Everclear. Though the song departs from the sterotype in that the "soccer mom" in the song has something of a checkered past and this is not necessarily part of the stereotype.