From cuda document
http://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#driver-api
http://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#versioning-and-compatibility
It seems cuda driver is libcuda.so which is included in nvidia driver and used by cuda runtime api
Nvidia driver includes driver kernel module and user libraries.
Cuda toolkit is an SDK contains compiler, api, libs, docs, etc…