The Pandora Battery comes in two sizes to fit into the battery holder of the Playstation System Portable (PSP). The Fat variant will fit into the Fat and Slim PSP. However, the battery back cover of the Slim PSP won't be able to close and you'll need to spare a hand to hold it in place. This will therefore incur a certain disadvantage in the enjoyment of using your PSP as you are then short-handed. The Slim variant will only fit in the Slim PSP.
Assuming you asked a question on why the need to use this battery when the maker's battery is functioning as expected, the answer is to expand the capabilities of your PSP. In simple terms, a Pandora battery is a special battery to boot a PSP into service mode. Service mode is unlike regular mode as you'll then be able to change the characteristics of your PSP through its firmware, as well as downgrade or upgrade your PSP. Some of these batteries are capable of changing state from Pandora to regular and vice versa. They are either triggered by a press of a button on the battery or via software.
Custom firmware is flashed or installed onto the PSP's motherboard or heart via a memory stick. This firmware is similar to the maker's official firmware except that it has the ability to run homebrew applications, emulators and other functions not available in a regular PSP. Emulators allow third party games to run on your PSP. Although it may be easier to purchase a pre-made memory stick from reputable sites, you can also create your own memory stick. Numerous sites offer guidelines and tips to carry out this task.
Before you think about implementing any changes to your PSP, check its compatibility against its model. Newer models of PSP may not allow custom firmware to run on them.
Since there is some change to the device's firmware, an often asked question is whether this is legit. Some reviews are of the opinion that it is as there is no breach of copyrights as well as running illegal software.