Nine-volt batteries are the "little square ones" used in electronic devices that require more power than the smaller 1.5V power source., but less actual power. This is because the battery is actually made up of six tiny 1.5V cells hooked together to boost the available voltage.
A 9V battery will not last as long as six 1.5V AA size batteries under the same current load, but they take up much less space. This makes them the battery of choice for many devices where space is at a premium.
Low Current Applications
Open the battery cover on most transistor radios and you will find a 9V battery inside. The radio does not require much current at one time, so the 9V battery is well suited to the task of running it. The low-amp load takes many hours to discharge the six cells inside the battery.
Smoke alarms often use 9V batteries. The device draws very little current and that's why the battery typically lasts a year, even though the detector is operating all the time.
Backup Power Source
The 9V battery is a good backup electrical source in case of power failure in many small electronic devices because they have a shelf life of several years.
9V Toy Batteries
You can find 9V batteries in toys such the hand-held radio control module of an electric car. You will find them less frequently in the motorised part of the toy because they discharge much faster under high current load than AA batteries wired together. Small motorised toys use them due to space considerations.