A scatter chart draws a single point for each point of data in a series without connecting them.
An important distinction between a scatter series and a line series is that the scatter series doesn't require sorting because the mouse tracker for the tooltip is activated on each single marker. A line can be drawn between the markers by setting the lineWidth option to something higher than 0.
For an overview of the scatter chart options see the API reference.
In 3D charts, the scatter points also have a Z dimension. This is handled by a separate series type, scatter3D.