A process is simply a running program on your system. Your web browser, window manager, terminal window and X server are all processes that you interact with directly. Many other processes run in the background, such as web servers and other system tasks. A new process is created every time you type a command like `ls` or `pwd`, though such processes are typically short-lived.

Every process has a unique ID, called the process ID or PID. While every process running at any one time has a different ID, over time PIDs may be re-used.

Apart from the initial process (typically called `init`) each has a parent process from which it was created. For example, if you run `vi` from your shell prompt, `vi`'s parent process will be your shell. A process can have any number of children, but only one parent.

Each process runs with the permissions of some user and group, which apply when it accesses files and directories. Users and processes may only kill other processes that they own, with the exception of `root` who can kill anything.