libpqxx is a C++ API to PostgreSQL. It replaces the earlier libpq++ with a more Standard Template Library (STL)-conformant interface and makes more extensive use of C++ language features such as exceptions, templates, and strings.
Since libpqxx builds on top of the C frontend libpq, you will need a working libpq library on your system prior to getting started with libpqxx. This is similar to the situation with the older libpq++ interface.