A 9-volt battery, sometimes referred to by its original designation as a PP3 battery, is shaped
as a rounded rectangular prism and has a nominal output of nine volts.
Its nominal dimensions are 48 mm × 25 mm × 15 mm (ANSI standard 1604A).  
The nine-volt battery format is commonly available in primary carbon-zinc and
alkaline chemistry, in primary lithium iron disulfide, and in rechargeable form
in nickel-cadmium, nickel-metal hydride and lithium-ion.                                                                                                                                            Rechargeable nickel–cadmium (NiCd) and nickel–metal hydride (NiMH) batteries
of nominal 9V rating have between six and eight 1.2 volt cells. Lithium ion versions
typically use two cells (3.7–4.2 V nominal each). There are also lithium polymer
and low self-discharge NiMH versions. 

Orthographic View of a 9 V Battery

Applications

9v batteries are commonly used in smoke detectors, guitar effect units, pocket radios, and radio-controlled vehicle controllers. They are also used as backup power to keep the time in digital clocks and alarm clocks.

Clocks: Although several clocks already have built-in batteries used as a primary power source, 9-volt batteries are most often used as backup energy. These batteries are commonly used to maintain the clock display and provide enough energy to maintain the internal circuits, which are used to keep the proper time after a power outage. 

Walkie-Talkies: Walkie-talkies frequently require much more power than most household electronics, which is why AA or AAA batteries will not suffice. A 9-volt battery holds the same amount of charge as 6 AA batteries, making it a much more powerful battery for Walkie-talkies and handheld talk radios. These batteries are used to transmit the signals between the two devices, while simultaneously powering the microphone and speakers. 

Smoke Alarms: One of the most common uses for 9-volt batteries are for smoke alarms or carbon monoxide detectors. These batteries help keep the detectors active, even during power outages. It is recommended that you change these batteries every six months or so in case the power begins to dissipate over time. 

Technical specifications

The battery has both the positive and negative terminals on one end. The negative terminal is fashioned into a snap fitting which mechanically and electrically connects to a mating terminal on the power connector. The power connector has a similar snap fitting on its positive terminal which mates to the battery. This makes battery polarization obvious since mechanical connection is only possible in one configuration. The clips on the 9-volt battery can be used to connect several 9-volt batteries in series. One problem with this style of connection is that it is very easy to connect two batteries together in a short circuit, which quickly discharges both batteries, generating heat and possibly a fire. While this is a danger, the same thing can be done with multiple 9 volt batteries to create higher voltage (they can snap together). The wiring usually uses black and red wires, red for positive, and black for negative.

Inside a PP3 there are ordinarily six alkaline or carbon-zinc 1.5 volt (nominal) cells arranged in series. These are either AAAA cells, or special flat, rectangular cells. The exact size of the constituent cells varies from brand to brand -- some brands are slightly longer than others -- as does the manner in which they are joined together. Some brands use soldered tabs on the battery, others press foil strips against the ends of the cells.

Very cheap versions may contain only five 1.5 volt cells. Rechargeable NiCd and NiMH batteries have various numbers of 1.2 volt cells. Lithium versions use three 3.2 V cells - there is a rechargeable lithium polymer version. There is also a Hybrio NiMH version that has a very low discharge rate (85% of capacity after 1 year of storage).