To view the latest rankings, You can view the ratings here.
What is Elo?
The Elo rating system is a rating system that awards points to teams based on competitive play. Originally designed for chess, Elo has since been adopted for other sports and has been viewed as an alternative ranking system to FIFA's rankings. The general idea is centered around stealing points from your opponents and adding them to your own score. If you beat a strong team, you take a large number of points. If you defeat an inferior team, you take only a few points from them. (Wikipedia)
FIFA has their own ratings for the women's game. Why create Elo ratings?
Most people are familiar with the Elo ratings we use on the men's side and FIFA's women's ratings are generally similar, they just use a different approach. FIFA is actually a type of Elo ratings, as there isn't a "right" way to do compile them. However FIFA's approach to the women's ratings creates a noticeably different outcome than the method used for the men's Elo ratings. If there wasn't, there would be no need to have an alternative approach. These rankings are made to have a mirror to the men's rankings, without the headaches that FIFA institutes in theirs.
It should be noted that FIFA men's ratings are done different than the women's. I'm not 100% sure why they wouldn't use one system for both, as it undermines at least one of the ratings, if not both, but not having a universal approach. Regardless, I'll refer to these ratings here as Elo because they're more in line with the commonly accepted version.
How are FIFA and Elo different?
While the top ten are essentially the same, the gap between teams is different. Number ten rated Canada is 230 points behind the US in FIFA's rankings, much smaller than the 400 gap of USA and (#10) North Korea. Equatorial Guinea is probably the largest standout, rated 32nd in Elo and only 50th in FIFA. (A little more on Equatorial Guinea can be found in the next answer.) Also the number one slot has changed hands more times in Elo ratings than FIFA's, due to the difference in how things are weighed.
The specific approach between the ratings are fairly different. For example, FIFA rankings can actually take away points from a winner if they do not win by enough, which Elo does not do. There also isn't a difference in winning 6-0 as there is 40-0 in FIFA's ratings. While the point behind doing so is fair and makes sense, it produces for a different outcome when looking at the final rankings.
Additionally there are odd restrictions to FIFA's rankings on the women's side. First, it only reaches back to 2003, despite having most of all international games on their site. Secondly, there is a stipulation that if a country does not play a game within eighteen months, they immediately drop to the bottom. As I write this, Argentina has recently dropped 99 spots to last place. (Technically they are not rated although they would be 133, after Botswana at 132.) Although apparently this is only temporarily, as Argentina are no strangers to the quick maneuver.
What are limitations of these Elo ratings?
Probably the biggest point for discussion is the base rating for a country. Every team has to start with a rating and this can range greatly. On the men's side, Elo ratings range from 1800 (great starting spot) to 600 (very bad). After a few hundred games, the starting point is relatively pointless and would only change a rating 200 games later by peanuts. However for teams that are low on games played, it does have a larger influence.
To give a clearer example, US women's team has played around 600 games, giving them a rating of 2176. Their base rating, before they played any games, was 1600. If I drop their base rating of 1600 to 600 their overall rating only drops to 2161. After 600 some odd games, their rating gets finalized.
An example of a country that hasn't played several hundred games is Equatorial Guinea. They have at Elo rating of 1231 over 44 games with a base rating of 900. Their rating of 1231 puts them 32nd in the world, but FIFA has them at 50st. FIFA has given them a lower base rating than a 900. If we move their base rating to 1200, they move to 1272, 29th overall. So to be honest I am not quite sure why Equatorial Guinea is so low in FIFA rankings. But all that to say, base ratings were tricky to finalize but ultimately not a big deal for countries who have played at least 50 games, which was most teams.
The only other hurdle I ran into was the K factor that Elo ratings use. The K factor weighs each game in importance. A friendly is basically worth one third of a World Cup game and everything else between that varies. (FIFA also weighs games less than Elo.) Again, there's reasoning behind doing so, it's just going to change the outcome if the math is different. (Read more about them Elo game weights here.)
How do the women's Elo ratings compare to the men's?
It is no secret that the men's game has more depth than the women's side and now we can see that more concretely. The top forty teams are separated by 400 points on the men's side, while the women are almost at 1000 points from USA to fortieth ranked Ivory Coast. Even inside the top ten we can see the disparity as USA is 400 points higher than North Korea.
However the women's Elo rankings are designed after the men's so they can be somewhat comparable. The lowest base Elo rating is 600, despite some 40 teams on the women's side being south of 600. The maximum base rating for the women's side is 1600.
How do Elo ratings compare between different generations?
Elo ratings are a system to show how dominate one team is at one given time. We can't directly compare the men's game to the women's as the landscapes are vastly different but even over time, it is tricky to compare the 1999 USWNT to the 2015 squad. Both teams get over 2100 points but it is in reference to their competition. For example, if the rest of the world - for whatever reason - became significantly worse over time, then a 2100 against weaker teams would not be the same as a 2100 when the rest of the world was better. Of course it is tough to say objectively if teams are better or worse over time, but it should be kept in mind when comparing two different periods of time.
How many games do you have in your database?
Currently I have 8677 games that start from April 1, 1920 to July 31, 2016.