Cartridges are a form of media used for playing games. They contain ROM files, and can also include additional hardware for video game systems to use.
One of the distinguishing features of cartridges is their ability to expand existing hardware. Many cartridges on some systems included long-life batteries for saving games, and some handheld game cartridges have included things such as gyroscopes. However, recently, cartridges have been somewhat inefficient compared to optical media, containing much less data per unit. That being said, the recent plummet in the price of flash memory has allowed cartridges to be the media format of choice for the recent Game Boy systems.
Cartridge-like devices were used as early as 1972, when the Magnavox console featured removable circuit boards. However, cartridges didn't truly take off until later in the decade with the Atari 2600 and other systems of the same generation. For the next fifteen or so years, virtually every major console used cartridges, including Sega's Master and Genesis systems and Nintendo's NES and SNES.
Cartridges started being phased out in the fourth generation, as various consoles incorporated CD technology or had CD-ROM add-ons. However, cartridges were used as the main format for the Nintendo 64, despite the Saturn's using CD-ROM technology instead. Although cartridges could only contain a fraction of the data of CD-ROMs, their lack of moving parts made lad times relatively quick. However, as the sixth generation came, even Nintendo would abandon the cartridge, instead opting for optical media.
Virtually all notable handheld gaming devices that didn't have built-in games or download them online have used cartridges. This is due to their small size encouraging portability, as well as their quick load times being ideal for quick bursts of play. Furthermore, their small size has kept manufacturing costs low, a traditional problem with console cartridges.