CUDA (Compute Unified Device Architecture) is a computing architecture developed by Nvidia, that allows programs to process algorithms in the graphics card GPU, instead of the computer's CPU.
Programs that make use of CUDA specially design some of their algorithms to make use of the GPU through Nvidia's framework. The program can then run both in the CPU and the GPU. Algorithms making use of CUDA will be sent to the GPU for processing, while algorithms that do not will be normally processed by the CPU.
CUDA is already being used by many systems. Particularly those that can take advantage of a GPU superior ability to perform floating-point calculations. These include scientific simulations and data processing, Distributed Computing and some encryption/decryption algorithms.