CDK is the company name. Stands for Cobalt (marketing firm that got bought and has since been spun off), Dealer Services (CDK used to be a subsidiary of ADP called ADP Dealer Services), and K (Kerrigan, the international arm of the business, it was spun off).
Should just rename it back to Dealer Services at this point.
DMS, as someone already mentioned, stands for Dealer Management System.