Unix Multi-processing

The major components of OS is
1. Process management
2. Memory managent
3. I/O Management
4. Scheduler
etc..

The scheduler is a process that assigns the CPU to each process a certain interval of time ( of the order of micro secs). Thusevery process gets CPU and performs its task. But at any given time in a single cpu machine, only one process would be running others will be waiting for CPU or for other resource. There different scheduling mechanisms such as round-robin, priority base etc.
Hopes this gives an idea.
Thanks & Rgds
rk