What is the extent of truth to the wage gap between men and women in America?

It seems to me that there is a lot of misinformation on this issue from both sides of the coin. I for one, accept that there is a discrepancy going on regarding equal pay between men and women. But I think there could possibly be other factors at play here. Such as job roles, experience, amount of hours worked per week etc.

When we talk about a gender pay wage gap are we talking about a man and women both with similar experience, working the same hours, doing the same job at the same company etc?

And how are the statistics calculated? Is it one general overal nationwide statistic? Are they separate statistics based on each sector and occupation?

Don’t get me wrong here, I’m not in any way cynical on this issue or anti-feminist by any means. But I am curious to the facts of this situation. And is the United States the only western nation where this is occurring?