Donut (or Doughnut) Charts are somewhat relative to Pie Charts, as both have the same restrictions, capabilities and purposes of usage. They can have an only series, an only axis, they are not good at displaying the data with a large number of points. There is an only difference between Pie and Doughnut Charts: a hole in the middle, which the second one has. It can be used for holding some text, for example.
This chart shows that something whole which consists of bonds and stocks; bonds and stocks are, in their turn, divided into a number of smaller categories. Five points that belong to the “bonds” group are colored in shades of green and other seven points of the “stocks” group are colored in shades of blue. The hole in the middle of the donut is useful here: there are two interactive labels that function like legend items: when the “bonds” label is being hovered or clicked, all points that belong to “bonds” become highlighted or exploded/unexploded; the same is with “stocks” label.