Plot styles are a mechanism for storing the contents of the plot options tabs. For example, you can save plotting parameters good for viewing XANES data and another set for viewing the full EXAFS spectrum. You can save any number of sets of parameters. When you save a set of parameters, you are prompted for a name. After that you can select a parameter set by name. These parameter sets are saved to disk, thus persist between sessions.
You can discard a plot style by double clicking on it.