There are the following four principal events that cause the processes to be created.
Generally, there are some processes that are created whenever an operating system is booted. Some of those are foreground processes and others are background processes.
Foreground process is the process that interact with the computer users or computer programmers.
Background processes have some specific functions.
In Unix system, the ps program can be used to list all the running processes and in windows, the task manager is used to see what programs are currently running into the system.
In addition to the processes that are created at the boot time, new processes can also be created.
Sometime a running process will issue the system calls just to create one or more than one new processes to help it to do its work.
User can start a program just by typing the command of the program on the command prompt (cmd) or just by doing the double click on the icon of that program.