The software is free to install and use; you pay only if you want to access hundreds of different girls.
It uses a Push technology (you get a new girl a day right on your desktop).
However, the software contains also an engine which manages the sequence of MPEG videos, so that, according to what you ask her, she might behave differently.
Virtual Fem is really hard-core, using videos of real girls.
Linux: Some Twinity members have attempted to run Twinity on Linux under Wine with various degrees of success.