Stream Processors -- Like nVidia's "CUDA Cores," AMD's Stream Processors are a more central component of the GPU. Stream Processors and CUDA Cores are not linearly comparable due to vast architectural differences, but can be thought of as similar when it comes to the primary function of each component. Stream Processing is focused intensely on parallelism of datasets to ensure efficient processing when performing tasks that are better-suited for parallel processing. These tasks can include aspects of audio processing (stream processors are used in DSP-equipped devices -- digital signal processors
Each GPU can contain hundreds to thousands of Stream Processors. Architecture changes in a fashion that makes cross-generation comparisons often non-linear, but generally speaking (within a generation), more Stream Processors will equate more raw compute power from the GPU. The processors are responsible for processing all the data that is fed into and out of the GPU, performing game graphics calculations that are resolved visually to the end-user. An example of something a Stream Processors might do would include rendering scenery in-game, drawing character models, or resolving complex lighting and shading within an environment.