Bubble charts are like single-series scatter charts, with one additional dimension as each bubble's size. They can be seen as (x, y, z) points displayed on a cartesian plot area, where (x, y) are the 2D coordinates, and z is each bubble's size. Bubbles are usually normalized, i.e. limited to sizes between a minimum and maximum value.
Our US Sales demo charts show a bubble at the intersection of each year value between 2000 and 2006 and the number of products sold in US in that year. The horizontal X baseline appears here as a category axis, with regular intervals for each year value. But bubble charts use also often a linear baseline, with X values distributed at random intervals. Main difference when compared to scatter charts is each bubble gets a different size. This is the third dimension, as an additional measure. In this case, some products may be more "valuable" than others, when same quantity has been sold. This is one single series and all bubbles get the same color.