Spice is a program developed by the EE Department at the University of California at Berkeley for computer simulation of analog circuits. In its original form you tell Spice what elements are in the circuit (resistors, capacitors, etc.), and then enter the circuit diagram as an ASCII file showing what nodes each element is connected to. Every node is assigned a number, and there is always a ground node, which is Number 0.
You then tell Spice what information you want -- bias conditions, frequency response, and/or transient response. Spice does the circuit analysis and puts out an ASCII file with the information. Using Spice is not very intuitive to use because the input is an ASCII file rather than a circuit diagram, and the output is another ASCII file rather than a graph. Several companies have developed graphical user interfaces for Spice, which make it much easier to use.
One of the most popular is PSpice.