How is video playback done on a computer? It's obviously not relying purely on the CPU, since video playback continues when a user performs another activity, such as typing into a YouTube comment field. This task seems further complicated in the case of streaming video, where data presumably needs to be transferred from the network interface to the video/graphics controller.
EDIT: I should have made clear that I understand that the CPU switches among multiple processes and threads all of the time. My question was meant to be more specifically about video playback. Is the video processing done on the graphics chipset/card? Do those typically offer buffering? Or can it all be handled by a single-core CPU with time to spare for other tasks, or am I wrong to think that low-end CPUs can play back video without (not-network) delays?