Execution stream is a sequence of instructions performed in a process state. Only one thing happens at a time within a process. No, it's both more and less. Less - one program may use several processes, e.g. cc runs other things behind your back. Easier to write some parts of OS, but many other things are hard to do.
Most systems allow more than one process.
They are called multiprogramming systems. First, have to keep track of all the processes. Scheduling information Accounting and other miscellaneous information. OS must make sure that processes don't interfere with each other.