What is a process?
Except the kernel, Everything on a Linux system runs in the context of a process. Process can be loosely defined as a running program. Every process on a Linux system has a unique Process ID (PID). Users and programs use this PID to communicate with the process.
The key command to find out various processes and their properties on a Linux system is ps.
E.g. the following command shows all processes currently running on the system:
# ps -e