A Fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations. It is not intended for use on an out-of-control fire, such as one which has reached the ceiling, endangers the user (i.e., no escape route, smoke, explosion hazard, etc.), or otherwise requires the expertise of a fire brigade.

Typically, a fire extinguisher consists of a hand-held cylindrical pressure vessel containing an agent which can be discharged to extinguish a fire. Fire extinguishers manufactured with non-cylindrical pressure vessels also exist but are less common. 

There are a variety of fires. Each fire type, cannot treated the same way that, is why we have to use different Fire Extinguishers for different fires. For example, a water type Fire Extinguisher cannot be used for an electrical fire. Fire Extinguishers work on the principle that by isolating Oxygen from, the fuel and ignition; the Fire Triangle is effectively broken. Hence, the fire stops immediately. However, Fire Extinguishers are supposed to be used for small fire or fire in an infant stage. For large fires, Fire Extinguishers they might not be useful. So in a sense, you can call Fire Extinguishers as Automatic Fire Extinguishing Systems, useful for fires in their infant stage. But the detection will, remain manual or visual; as seen by the person near the fire.