The Raspberry Pi hardware, essentially being a small and cheap circuit board, has been covered in different articles quite extensively, so I will focus here on the board from software developers perspective when creating IoT prototypes.
As RPI is not an ordinary embedded device, SDK can be a misleading term for it. To be functional it needs an operating system. The most common OS is Raspbian which is derived from Debian/Linux. As Raspbian is a full Linux distribution, it contains the same tools that can be found from the original Debian. For development needs, additional libraries need to be installed. For example GPIO pin controlling has its own libraries.
While RPI itself is a final product, there is not any cloud service for it like some other embedded devices may have. But on the other hand, we can connect it to any cloud service of our choice or to other external services, such as own custom backend. The lack of a proprietary back-end or cloud service is an approach for some other embedded device manufacturers as well, but other vendors have decided to provide some kind of a cloud for the devices, sometimes even without any possibilities to change it. We prefer to have open possibilities for the back-end even with the cost of a slower deployment of the IoT prototype setup. This way the prototype back-end service can be better utilised for the final system as well, where the hardware may be a different one.