ROM = Read Only Memory. Essentially, it's where the OS for your device resides. If you have a phone, you've got ROM. I think what you're asking about are alternate (as in different than the mfg supplied) ROMs. For most Android phones, there's a developer community that makes alternate software for that device that may increase performance and/or battery life and add features. For Android, the way to install alternate ROMs/Kernals is to root the phone. Rooting is nothing more than obtaining permission within the OS to alter the basic OS files. The best advice would be to do your research within the forum of the device you own or plan to own since the nitty gritty details change from device to device.