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Android is an operating system based on the Linux kernel, and designed primarily for touchscreen mobile devices such as smartphones and tablet computers.
SpecsUpdate
Pros
Cons
Con App permission issues
Outdated: It is very unlikely that you buy a Lollipop phone in 2020.
Any users from Android M onward are not affected by this issue.
Permissions are only granted via a pop-up when the app requests.
The application permission system used requires requesting access to more user data than should be needed to run an app. For example, in order to enable matchmaking in games the app needs access to contacts list. Additionally, since Google doesn't enforce requesting only the permissions necessary for an app to function, a culture of requesting permissions that aren't in any way needed for the app to function has emerged.
Con Fragmentation and security flaws due to negligence
Most Android phones seem to be released on a "Let's release a few software updates until we release the next phone, and never push out an update again." For example, In May 2014, 8.5% of phones were Kitkat, 60.8% were spread out between three versions of jellybean (with bias towards older versions), and the another 20% spread out over ICS, Honeycomb, Gingerbread, and Froyo.
Google no longer maintains Android 4.3 or prior and serious security exploits remain in those versions. Device manufacturers are also taking no action, which leaves almost a billion Android users exposed.
You may also encounter lingering and unfixed bugs, glitches, etc. as a result of the manufacturer's and Google's lack of interest in older versions.
Some flagship phones are well supported into the future, but the fact that your phone won't be supported by its manufacturer and/or Google is a consideration you must make when purchasing an Android phone.