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Lineage OS is custom OS/ROM for Android devices. Lineage is based on (AOSP) Android Open Source Project code.
Lineage OS developers add features, fixes, and improvements that Google didn't include
SpecsUpdate
Pros
Pro Provides granular control over personal data
Compared to stock Android, Lineage OS gives a lot more control over user's privacy. There's a permission manager that can be used to prevent certain apps from accessing certain information, a blacklist for blocking any unwanted calls or messages for specified numbers and notification filter that can be used to remove certain apps from popping up in the notification area.
Pro Lots of customization options
The main strength of Lineage OS is the customization options that it brings to Lollipop.
For example, status bar items such as clock and battery can be customized, pending notification count can be displayed in the status bar, Quick Settings tiles can be reorganized. It can be set up to adjust brightness by sliding across the status bar or access Quick Settings by pulling down from the edge of the screen. On devices like the OnePlus One there's the ability to switch between onscreen and offscreen navigation buttons. Layout of the onscreen buttons can be changed.