This is the reasoning for my question: I read this in a text book Each CPU (or core) can be working on one process at a time.I'm assuming that this used to be accurate but is no longer fully true. How does multi threading play into this? Or is this still true, can a cpu core on linux still only work on one process at a time? 
Can a single core of a cpu process more than one process?
cpu;multithreading
A single CPU handles one process at a time. But a process is a construct of an operating system; the OS calls playing a video in VLC a single process, but it's actually made up of lots of individual instructions. So it's not as if a CPU is tasked with playing a video and has to drop everything it was doing. A CPU can take on the task of playing a video  switch over to checking for keyboard or mouse input  draw some stuff on the screen  check to see if devices have been attached in any known port  and so on. All within the blink of an eye.Modern computers excel at multi-tasking. This is why you can launch a video in VLC and have it play continuously even though your computer is doing 100 other odd jobs at the same time.