PlotStyle is an option for Plot and ListPlot that specifies the style of lines or points to be plotted.
PlotStyle -> style specifies that all lines or points are to be generated with the specified graphics directive, or list of graphics directives.
PlotStyle -> , , ... specifies that successive lines generated should use graphics directives , ... . The styles must be enclosed in lists, perhaps of length one.
The are used cyclically.
Styles can be specified using graphics directives such as Dashing, Hue and Thickness.
See The Mathematica Book: Section 1.9.3 and Section 2.9.3.
See also: Graphics, TextStyle.