Words are inadequate

um. i'm not really sure what exactly SPICE does... are you at all familiar with hardware design languages? they're a way of simulating your circuit in code. Verilog (the one we use most, although we've got support for a couple others) is entirely event-driven (ie, changes in signals occur at time #blah), so you have to come up with a timing diagram for the input signals.

basically, the product we've got (TestBencher Pro) consists of a timing diagrammer (you draw waveforms and it exports them into Verilog code, with delays and all that fun stuff) and a fairly powerful Verilog simulator. There's more to it, of course, but that's what I'm currently capable of using properly.

I think the timing diagrammer is capable of importing SPICE analog waveforms, but I haven't played with any such things yet.