pgf stands for `portable graphics format'. It is a TeX macro package
that allows you to create graphics in your TeX documents using a special
pgfpicture environment and special macros for drawing lines, curves,
rectangles, and many other kind of graphic objects. Its usage closely
resembles the pstricks package or the normal picture environment of
LaTeX.

Although pgf is less powerful than \pstricks, it has the advantage that
it can generate both PostScript output and pdf output from the same
file. The pgf package works together both with dvips and pdftex. In
particular, packages that rely on pdftex or pdflatex (like some packages
for creating presentations) can be used together with pgf.