Glossary
Signal
An asynchronous message sent to a process, like SIGTERM or SIGKILL.
Signals are the kernel's way of tapping a process on the shoulder. Each has a number, a name and a default action: SIGTERM (15) politely requests termination, SIGHUP (1) traditionally means "reload your configuration", SIGINT (2) is what Ctrl+C sends.
Programs may install handlers to react gracefully — flush data, close files, reload config. Two signals cannot be caught or ignored: SIGKILL (9) and SIGSTOP, which is why kill -9 always works and why it never lets the program clean up.
See it in practice
$ kill -l | head -3Lists the signal names the system knows. Sending one is kill -NAME PID, and the default is SIGTERM.
Where you'll meet it
Terminate processes politely or forcibly — and understand what -9 really does.
jobs, & and nohupRun things in the background, juggle them, and keep them alive after logout.
ProcessA running program, with its own ID, memory and environment.