Control Panel
This is effectively the computer or CPU of a security system. It must be programmed by your installing company to respond appropriately to signals from the alarm sensors, depending upon the "armed" state of the system, i.e., On&Off - or in alarm tech terms: "Armed&Disarmed".

Keypad
This is the interface you use to change the state of the system to "armed or disarmed", typically by entering a specific numeric code. It also provides a display which shows you the current state of the system: on/off, alarms, power status&various trouble conditions. The display can be either a series of lights, an actual text display, or a combination of the two.

Standby Battery
This is typically a rechargeable battery which provides operating power for your system in the event of an electrical failure to the system. Normally it is recharged automatically as long as electrical power is available to the control panel. It also provides additional power for the operation of bells and or sirens during an actual alarm condition. Lastly and possibly most importantly, it provides a large percentage of the electrical power filtering to your control panel and detection devices, thereby increasing their reliable life span and their stability during momentary electrical power fluctuations.

Siren
This is a VERY loud sounding device which sounds during an alarm condition. Typically, they will generate different sounds to allow you to distinguish between a burglary alarm and a fire alarm. Many sirens contain integral "siren drivers" (see below). A quality system will have at least two sirens, one each inside&outside of your premises, thereby alerting you, your neighbors, any passersby, and the would-be burglar of the alarm.

Siren Driver
This is either a separate or built in component of the control panel which generates the different tones which are emitted by the sirens. Several versions are available which can produce separate tones, broken tones or actual voice messages in the event of an alarm, i.e., fire or burglary.

Digital Dialer
Also known as "digital communicator": This is typically an integral part of the Control Panel which allows the transmission of alarm signals from your premises to the alarm company central monitoring station. Some older systems and some commercial fire alarm systems utilize separate digital dialers which are not integrated with the "Control Panel". It is effectively a modem which contacts the alarm company's modem in the event of an alarm or other supervisory conditions, i.e., low battery, a.c. power failure, etc.. It utilizes an existing telephone line at your location to accomplish this and is completely transparent to the phone or phone system unless a communication event occurs.

GSM or Cellular Communicator
This is a device which allows for an alternative path for alarm signals to reach the central monitoring station in the event of a telephone line failure or purposeful tampering with the telephone lines by a burglar.
Extremely fast communication from the premises, relatively low cost units which are typically invoiced to you by your alarm company rather than a separate cellular company, normally transmit alarm signals simultaneously with telephone line transmission / Disadvantages - Can only be used in areas which have established service with a control channel cellular provider, Can not provide regular voice communication over your existing phones, transmit fairly generic alarm information, i.e. (burglary alarm) vs. (burglary alarm front door).

Transformer
This is the primary power supply for the control panel. It is typically a plug in type unit which is plugged into a standard wall receptacle. It supplies the normal operating power for the control panel and the charging power for the standby battery. Normally it is found near the control panel location. Frequent locations are, behind the clothes washer, in a utility room or garage. Most transformers have a top screw which allows them to be attached to the center screw location on the receptacle plate to help avoid accidental unplugging.

Power supply
This is normally integrated with the Control Panel, however some systems may contain an auxiliary power supply to provide power to a large number of detection devices. All auxiliary power supplies should contain an additional standby battery and an additional plug in transformer.

Zone expander
This is an add on device to the control panel which allows a further division of the detection devices into additional "zones" (see below). Different manufacturers refer to these by their own terms, and each manufacturer provides different numbers of zone capacities on their zone expanders, but they all do the same thing. Several systems allow zone expanders to be remotely located from the control panel allowing for simpler, more straightforward system wiring; Example, all windows in the bedroom area can be wired to a zone expander in a bedroom closet, rather than to the control panel location in the utility room.

Door&Window contact
(Also referred to as "switches") These are typically magnetically activated switches consisting of two components - the actual electrical switch and a matching magnet. They can be recessed into door&window frames with the matching magnet placed in an adjoining location on the movable part of the door or window, but are often surface mounted depending upon the type of door or window and whether or not the system is installed during or after construction. Other types of contacts are available and are sometimes used if more practical for the application - these are usually mechanical type switches which are physically depressed by the door or window when it is closed.