http://www.t-shirtforums.com/graphics-design-help/t55585.html
Should cover you for spot!
From my understanding, simulated separations use software to emulate a certain finish, normally with respect to 4 colour (CMYK) process printing. These "simulations" can separate into more than 4 colours, (say upto 9 colours, for example), giving you vibrant pop on your finished product. The final output from the software does often need tweaking to get the desired finish, though - not just a case of running software.
When ready to print, these are printed wet-on-wet to get the desired colour range, often with a specific pantone pallette of ink colours
Look up programs like separation studio on youtube for more info and go to their website to download a trial. You may also need a RIP for your printer to get the halftones that these programs output
Richie