CSS= SOURCE ENGINE YEAR MADE 2004
CS1.6= GOLDSOURCE ENGINE YEAR MADE 1998
Interestingly they do share their origins. They both come from the Quake engine way back when. Just before HL1 was released they forked the project into 'Source' and 'GoldSrc', from the practice of burning master CDs which are made of gold. Since the HL1 engine was the one being released it became GoldSrc.