Plot "forms" are the instructions for what shapes to actually paint on the plotting surface given some positional data from a table. The most obvious (Mark) is simply to plot a marker with a fixed size and shape at each data position. However, there is a range of other things you might want to do, including error bars, vectors, contours, text labels, ...
The options provided are described, with the additional configuration information required, in the following sections. Not all are available for every plot type.