SDK = Software Development Kit. In general terms an SDK is just the tools and libraries needed to develop for some piece of hardware or platform, or sometimes a set of tools that use an SDK to interface with a piece of hardware.
SSDK = "Switch Software Development Kit", an SDK for certain ethernet switches. Usually SSDK refers specifically to Qualcomm Atheros' drivers which communicate with their hardware
QCA-SSDK = Qualcom Atheros Switch Software Development Kit, which is a driver system for certain Qualcom Atheros chipsets.