CST (Computer simulation Technology) questions. Curious

Hi everyone,
I just started exploring CST.
I am curious of this : "expert system"
Does anybody have a definition for this? how does it work?
kindly please explain. Thank you.
Thanks

An expert system is a computer program which contains a database of information on a particular field of knowledge. (Medical, for instance.) It can be designed to interview a human user by presenting a branching tree of queries, in an effort to diagnose maladies, or decide whether there's an emergency, etc.
How you respond to a question determines what question the expert system asks you next.
Computer: 'State symptoms please.'
You type 'sore foot'.
Computer: 'Is the affected area bleeding?'
You type 'No'.
Computer: 'Is the affected area on the skin or inside tissue?'
You type: 'near the skin'.
Computer: 'Is there swelling?'
You type 'Yes'.
Computer: 'Possible causes: bunion, bone spur, cancer... Shall I make an appointment for you to visit the foot specialist?'
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Now I realize those automated menus I talk to on the telephone are expert systems.
They ask whether I want to do this or that. I answer by speaking one word, and they go on to the next question by asking me which account, etc.

Thank you BradtheRad.
i now have a clearer understanding.