Bubble charts are an interesting way of showing data that has a third dimensional component. You map two dimensions along the usual X and Y axes, and then the third dimension is displayed as a shape, usually a filled circle; hence, "bubble") at the data point. The larger the shape the bigger the value.
Because we humans have great difficulty in comparing the areas of two similar shapes of differing sizes -- it's hard to see one area as twice another, for example -- it can be easy to distort the meaning of the data. This means that this kind of chart is best used as a quick visual hit of some interesting information, rather than a dense data-rich image.
The bubble chart in XtraCharts not only includes circles as the markers, but also shapes like squares, stars, triangles, and so on. You can specify the size of the smallest and largest marker for the chart (and again, this goes to the notion that information from bubble charts can be "manipulated" in some sense, hence that they're best for a quick visual blast of information).