The Saturn Outlook, GMC Acadia, Buick Enclave, and Chevy Traverse are the variations of the base GM Lambda product. Lambda is the generic term for any/all of the above. There's probably a link to a better discription I'm sure someone will find it and post it.
GM has recently started using greek nomenclature to describe the basic platform that their vehicles are built off of. Like hgpayne said, Lambda is the basic platform that GM's large crossover SUVs are built off of.