SCO is the common abbreviation of The SCO Group, which in turn is one member of a large group of corporate shells controlled by the Canopy Group. The very name SCO is a disinformation act, attempting to blur the difference from an earlier corporation, Santa Cruz Operation previously known and enjoying a fine reputation in the software world as "SCO". Santa Cruz Operation sold some inexactly particularized assets to Caldera, Inc. and then later "old-SCO" changed their corporate name to Tarantella.
The SCO Group picked up the abandoned SCO name, and through the subset of assets passed through Caldera, have pretended to be a 25 year old company.  To try to minimize confusion, writers often refer to this new entity by their stock exchange abbreviation SCOX   or SCOG or TSCOG. Because of the most famous litigation of our times, and TSCOG's use of SCO throughout their court papers, there are many opportunities to fall into confusion over the SCO name.
This is TSCOG's website: http://www.sco.com/
This is an "official" rebuttal page from Open Source Initiative: OSI Position Paper on the SCO-vs.-IBM Complaint http://www.opensource.org/sco-vs-ibm.html
For more information about SCO's lawsuit Against IBM, and the disinformation campaign they dish out the public, please visit www.Groklaw.net