PlotLabel is an option for graphics functions that specifies an overall label for a plot.
PlotLabel -> None specifies that no label should be given.
PlotLabel -> label specifies a label to give.
Any expression can be used as a label. It will be given in OutputForm. Arbitrary strings of text can be given as "text".
See The Mathematica Book: Section 1.9.3 and Section 2.9.5.
See also: AxesLabel.
Related package: Graphics`Legend`.