Custom ROMs are modified versions of the operating system for mobile devices, primarily based on the Android platform. They allow users to replace the stock firmware provided by manufacturers, offering enhanced features, improved performance, and a more personalized user experience. Custom ROMs can also provide updates to older devices that may no longer receive official support. Users often install custom ROMs to gain access to new features, better battery life, or a cleaner interface. Popular custom ROMs include LineageOS, Pixel Experience, and Paranoid Android. However, installing a custom ROM requires technical knowledge and can void warranties, so users should proceed with caution.