From the AP:
Women make only 80 percent of the salaries their male peers do one year after college; after 10 years in the work force, the gap between their pay widens further, according to a study released Monday.
The study, by the American Association of University Women Educational Foundation, found that 10 years after college, women earn only 69 percent of what men earn.
Even after controlling for hours, occupation, parenthood, and other factors known to affect earnings, the study found that one-quarter of the pay gap remains unexplained. The group said that portion of the gap is "likely due to sex discrimination."
It's not clear exactly what this gap means, but let's be clear about what it does not mean. It does not mean that two first year associates, one female and one male, hired by the same firm to do the same job will make different amounts. They will, in fact, make the same salary.
As for what it does mean, my best guess is that women tend to take jobs that pay less than men. For example, there are more women who take elementary school teaching jobs and more men who become engineers. The article says that this is due "sex discrimination." That's probably not the best way to characterize it, however, as it seems to imply the cause is mainly the decisions of senior male hiring people. In truth, I think there's a broader issue of how society views the role of women and men that is the root cause.