An execution stream is a sequence of instructions.
Process state determines the effect of the instructions.
Process is a key OS abstraction that users see - the environment you interact with when you use a computer is built up out of processes. The shell you type stuff into is a process. When you execute a program you have just compiled, the OS generates a process to run the program. Your WWW browser is a process. Organizing system activities around processes has proved to be a useful way of separating out different activities into coherent units.