Pynac is a derivative of the C++ library GiNaC, which allows manipulation of symbolic expressions. It currently provides the backend for symbolic expressions in Sage. In order to use Pynac, one needs to have the Sage library available.
The main difference between Pynac and GiNaC is that Pynac relies on Sage to provide the operations on numerical types, while GiNaC depends on CLN for this purpose.
For more information on how Pynac came to be, and the differences between Pynac and GiNaC, please visit the FAQ page.