Soul of Chogokin (超合金魂) or SOC, is Bandai's high end line of collectible robot toys. They embody the spirit of the vintage Popy and Bandai Chogokin toys of yesteryear. These toys are not meant for children, rather they are aimed at the adult collector with disposable income, looking to recapture a bit of their youth.
The line began as Bandai was winding down their Hi-Complete Model line. The last figure in the line was meant to be Mazinger Z, but they decided to turn that prototype into the first entry into the new Soul of Chogokin line. The line debuted in 1997 and was met with critical acclaim. As the line continued, Bandai refined their process and went back and re-did certain entries in the line, resulting in R or Renewal versions.
Soul of Chogokin toys come from a variety of shows and different eras in robot history. Some of the defining characteristics are that they all have a number (GX), all have some sort of metal content, and all represent the spirit (or soul), of the original character.
Wikipedia SOC Complete Listing
Bandai's official Soul of Chogokin Website