There are several methods of adding a label to a plot. You can use PlotLabel, Labeled, or interactively add text to a plot using Drawing Tools. For a description of adding text to plots using Drawing Tools, see How to: Add Text outside the Plot Area and How to: Add Text to a Graphic.
To add a centered label to the top of a plot, use the PlotLabel option:
You can control the appearance of the label by wrapping it in Style options:
To put a label underneath a plot, like a caption, use Labeled: