A running system contains many processes. They are programs running in the CPU one at a time, occupying memory and system resources. In a multitasking system, each process takes a specified fixed quanta of time in the CPU before switching to another process. Each process in the system can have different state: running, sleeping, waiting, etc.
To see what processes your system has, you need the command ps: The output is usually too long to be shown on one page, use pager less to help you:
ps aux | less
From the output, you can see:
The switches aux to ps means all processes (a), showing user information (u) and showing processes without controlling ttys (x).