Typically we use temperature, volume, and pressure to describe a large system of particles. We call such a system and all its thermodynamic properties an ensemble. The Wasser program uses four thermodynamic properties: pressure P, volume V, temperature T, and number of particles N. The values of three of these four properties can be fixed at one time to describe the water ensemble; the value of the fourth property results from the natural behaviour of the system.
For example, imagine a pot of boiling water. Put a lid on the pot, a special gas--tight lid that slides up and down inside the pot like a piston. This lets the temperature, the number of water molecules, and the pressure be fixed properties. The volume, however, is not restricted; the piston can slide up and down, so the volume can expand or contract. In contrast, now picture a container that is filled to the top with water and then sealed off. The volume is thus restricted to that of the container, the total number of water molecules is constant because the container is sealed. If we now fix the temperature, the pressure has to adjust accordingly.
In the Wasser program, the user can choose between two different ensemble setups: the NVT ensemble or the NPT ensemble. In the NVT ensemble, the number of particles N, the volume V and the temperature T can be set to desired values; the pressure will be a result of the system behavior based on your choices. In the NPT ensemble, the number of particles N, the pressure P and the temperature T can be fixed, and the volume will result from the system behavior.