Many mobile phone carriers and Android device manufacturers include additional programs and utilities that aren't part of the core Android operating system. These additions are sometimes called "bloatware" because they occupy a device's limited storage and they can't be removed. To eliminate bloatware, you must replace the version of Android on the device with a version that's not customized. Before you can replace Android, you must first run Android as the user without any security restrictions, called root, and then change the security setting that prevents you from replacing Android, called "NAND unlock."
On an Android-based device, configuration, user and data files are stored on internal flash memory, called NAND memory, which is read-write capable. Many devices also enable you to insert an external memory card that provides more read-write storage for user files, such as music, pictures and videos. The Android operating system, including many carrier- or manufacturer-specific additions to Android, are stored in read-only memory, or ROM. Since read-only memory can't be changed, you must replace the entire contents of the device's ROM with a custom ROM to change or remove any of the programs stored there.
When you power on an Android device, a small program called the bootloader looks at the device's security setting and then runs the Android operating system. If security is set to "S-ON," it locks the portion of the NAND memory that stores the Android configuration settings by loading it in a read-only format. On some devices, when security is set to "S-ON," the bootloader won't run Android unless the version in ROM is the official carrier or manufacturer version for the device. To install a custom ROM, you must NAND unlock the phone by changing the security setting to "S-OFF," also called "unlocking the bootloader."
When you run Android on your device, you run as a guest user. When a carrier or device manufacturer configures Android, it determines the settings that the guest user is allowed to change. For example, a core feature in Android is the ability to share your Internet connection with another device through tethering. Some carriers disable that feature and configure the security so that the guest user can't enable it. By default, the guest user does not have the ability to change the security setting to "S-OFF" and NAND unlock a device.
Rooting a Phone
Android has a built-in superuser called "root." Unlike the guest user, the root user has no security restrictions; it can perform any operation or modify any file on the phone. When you root an Android device, you gain the ability to run Android as the root user, which enables you to change the security setting to "S-OFF." For this reason, you must first root a device before you can NAND unlock it. Once the phone is rooted and NAND unlocked, you can then install a custom ROM to load a new version of Android on your device. It is wise to note that rooting your phone may void the warranty.
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