How does an alarm work? What is the differnce between a keypad and control panel?

Imagine the alarm system is the human body. The control panel is the brains, the keypads are the mouth, and all the other devices, such as contacts, shocks, motion detectors, etc., are your legs,arms, and other nerves. 

When you touch a hot pan, your fingers feel the sensation and sends a message to your brain that you are in pain. Your brain then sends a message to your mouth to screem ouch!, and you know to move your hand.

When someone opens a window on your system, it is the same as the pot on your hand. The sensor sends a signal back to the control panel, to say the the circuit is no longer closed. Now when the control panel receives the message, it like your brain, processes the message. Depending on whether or not your system is armed or not will change the message the control panel sends to the keypad. If the system is armed, and the window opens, the control panel will tell the keypad (the mouth) to start beeping and display which zone is in alarm. In addition, the conrol panel will tell the siren to sound as well (your screaming from the hot pot). If the system is not armed, the control panel will the keypad to display the open zone you are aware of it, but it isn't necessary to call for help. This is know as a fault or an open.

No matter which device is triggered it all operates the same way. The control panel interpretes the message and tells the keypad what to say. This way when you go over to the keypad you know which window is open, or smoke detector went off.