Your Android’s recovery image is normally responsible for flashing over-the-air (OTA) updates from your phone’s manufacturer (Samsung, HTC, Motorola, etc.), but the recovery image is even more important if you want to customize your Android with a custom kernel or custom ROM.
Kind of like dual-booting on a desktop computer, Android is split into multiple “images”, such as your “system image” where the operating system itself runs, and the “recovery image”. The recovery image is like a miniature operating system that is only responsible for flashing updates and related tasks.
Booting into the recovery image varies for each device, but usually includes powering your device off, then pressing a key-combination while holding the power key. The stock recovery image doesn’t let you do much — it’s not supposed to. You can flash updates that you got over the air, or updates that you downloaded and copied to a specific location on your Android. That’s about it.
Booting to a custom recovery image, like ClockworkMod Recovery (CWM) will let you flash just about any update .zip, backup your system or restore a previous version, “fix permissions”, and so much more! Installing a custom recovery image isn’t that difficult, as long as you’ve unlocked your bootloader, but again, how you do this varies by device.
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