Range (sometimes referred to as true range) is an instantaneous measurement of the distance between the transmit antenna on the SV and receive antenna on the vehicle. If it were possible imagine a tape measure stretched out between the two antennas, this measurement is the true range.
Pseudorange is the range that a GNSS receiver measures between the transmit antenna on the SV and its' receive antenna. It differs from the true range due to many physical effects. (Please refer to FAQ11129 for a complete list of effects that are considered when calculating changes in the pseudorange).
In the real world pseudoranges are noisy by nature since they are subject to stochastic and non-stocastic errors. In the simulated world pseudoranges are determinate and are clearly defined by a set of equations.