Manual for Package pgfplots
2D/3D Plots in LATeX, Version 1.18.1
http://sourceforge.net/projects/pgfplots

The external library offers a convenient method to export every single tikzpicture into a separate .pdf (or .eps). Later runs of LaTeX will simply include these graphics, thereby reducing typesetting time considerably.

This library is documented in more detail in Section 8.1 “Export to pdf/eps”.

The external library has been written by Christian Feuersänger (author of pgfplots). It has been contributed to TikZ as general purpose library, so the reference documentation along with all tweaks can be found in the PGF/TikZ manual (Section “Externalization Library”). The command \usepgfplotslibrary{external} is actually just a wrapper which loads \usetikzlibrary{external} or, if this library does not yet exist because the installed pgf has at most version 2.00, it will load a copy which is shipped with pgfplots.