Each circle is a country, it's size is related to it's population. The x-axis shows the kilocalories per capita for that country and on the y-axis grams of fat per capita. The colours denote different geographical regions.
Slide your mouse over year to change the year and see how the countries move.
Hovering over a country will show you which country it is.
The graph was initially designed to linearly interpolate data for years between data points, but it only does this going forward, which is why some countries don't move until their first available data point. It also causes Yugoslavia to drop off the graph because it had a negative change for it's last two years and so continues on a downward trend.
It's also really complicated when countries split up, for example when the USSR spilt into Russia, Ukraine and the other countries. Gapminder does some calculations to try and ensure that the same land areas are represented throughout their data despite border changes.
Adapted from Mike Bostock's Nations motion chart.