SPICE is a computerised modelling technique developed by Berkeley University in the 1960's - originally for the design of integrated circuits, but has found universal use for other applications.

By entering details of the circuit, you can check the frequency and phase response of the circuit (AC analysis), and check the circuit response over a set period of time (Transient analysis). There are other analyses to check the effects of temperature variations, and noise.

SPICE allows circuit designs to be tested "on paper" before they are actually prototyped - a great help in the design process, which also allows component stresses to be checked allowing the right items to be selected for reliability.   For a more in depth look, check out the University of Pennsylvania EE department overview...

The best way to understand how it works is to pick up a simulator and try it out. Some simulators come with models for valves and transformers, but many don't. I'm collecting models of both, and you can check them out in my SPICE models page....