Forbes uses a specific system to decide who to include. From a preliminary group of over 250, it ultimately chooses the 100 from eight categories/power bases: billionaires; business; celebrity; finance; media; philanthropy and NGOs; politics; and technology. To determine the category ranking (and eventually the overall ranking), each woman is measured by "money, media momentum, spheres of influence, and impact." A complete explanation can be viewed here.
As with any systemic ranking of people, there are inherent issues with Forbes' list. Many of the women, for example, are famous in tandem with their husbands and might not appear solo — which is not to say they aren't incredible in their own right, but their influence works in tandem with another's (Bill and Melinda Gates for example). Many also come from privileged families — even Laurene Powell-Jobs is listed at number 29 with an "and family" attached to her name. And, perhaps we're biased, but the lack of artists, authors, and journalists on this list is a bit of a bummer. Still, it's always a wonderful reminder to see the massive countries, corporations, and philanthropic organizations that are helmed by truly influential women. (Forbes)