Tom Halfhill has put together a very good overview of CUDA.
http://www.nvidia.com/docs/IO/55972/220401_Reprint.pdf
The official Nvidia documation for CUDA including a QuickStart Guide, Programming Guide, and Reference Manual for all versions and supported platforms can be found at
http://www.nvidia.com/object/cuda_develop.html
The Nvidia CUDA C Programming Best Practices Guide can be found at
http://developer.download.nvidia.com/compute/cuda/2_3/toolkit/docs/NVIDIA_CUDA_BestPracticesGuide_2.3.pdf