Android is an operating system based on the Linux kernel, and designed primarily for touchscreen mobile devices such as smartphones and tablet computers.
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Pro Open source
The AOSP (Android Open Source Project) is the software stack for the Android OS. It is led by the open handset alliance which includes firms like Google, Acer, NXP, Motorola and many others.
The fact that Android is an open source software has led to many custom Android variants. It also is a big assurance of Android's safety in terms of privacy.
Pro Customize what apps perform what actions
Android gives users the ability to define which apps perform which action. For example, if an Android device has several browsers installed, the user can choose a default one which will start every time a link is for example opened from another app.
Pro Lots of hardware choices
There are about 18000 released devices that run on Android. This means that a user familiar with Android can have a large variety of devices from which they can choose.
Pro Integration with Google Services
For those that already use Google's services, like Google Maps and GMail, Android offers tighter integration with those services than any of the alternatives, and saves users the need to create extra accounts.
Pro Highly customizable
One of the massive benefits of Android being open-sourced are all the modifications you can make to the phone to customise pretty much anything. Some of the cooler things you can do are:
Pro Consistent back button makes navigation easy
Pro More integrated sharing options
Pro Desktop and lockscreen widgets
Desktop widgets provide quick access to the information you need quickly and access the most. These are available out of the box, and can be developed by anyone. Widgets on the lockscreen have been removed since the Lollipop update, but is available on 3rd party lockscreens such as AC Display and KK Locker.
Built in text to speech is provided by TalkBack for people with low or no vision. Enhancements for people with hearing disabilities are available as are other aids.
Pro Personal assistant with Google Now
Google Now is an intelligent personal assistant available within the Google Search mobile application for the Android and iOS operating systems. It's software that makes your life more convenient by giving you all the information you need at any moment.
Pro The most widely used mobile OS
Pro Built-in screen recording
Android 5.0.+ supports screen recording via a new api.
Android 4.4 KitKat supports screen recording accessible via an ADB command on unrooted devices.
Pro Great user experience right out of the box
Pro Customizable multiple "desktops"
Can customize the home screens with widgets (not just a grid of apps like iOS).
Pro Security updates
Android does frequent updates to keep users secure.
Pro Not bound to Google
It is fully working without the play store or the Google play services.
Pro Sideloading of apps
You can sideload apps.
You can root your phone to get full access.
Pro Supports plenty of peripherals
You can use external displays with MHL, HDMI, DP or miracast. It supports external storage devices like microSD cards or USB OTG devices. You can also use almost any kind of USB or Bluetooth input devices like keyboards, mice or game controllers.
Pro Gesture based interface
Since Android 10 you can use gestures to control the OS.
There are major versions of android however the most important components are modular and get frequently updated.
Pro Highly secure core system
Since Android is open source and developed worldwide, anyone can read the code and there are no government forced backdoors.
Pro Cheap phones
Android phones can cost little as $100.
Pro No Backdoors
Since Android is written by a worldwide community of developers it is very unlikly that backdoorss exist for any local governments.
Pro It's all yours
You can do anything you want with YOUR phone.
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.
Con Wide variety of phones forces variance in quality
While there are many great Android phones, there are also many Android phones that lack in both hardware and software. When purchasing Android phones, a consumer might want to do some research first - especially when buying on a budget. Not only do you have to worry about shotty hardware or poor quality testing, manufacturers tend to tinker with the software on phones. More often than not this leads to poor performance, unintuitive 'features', slower (or complete lack of) updates, and custom software that detracts from the user's experience.
Con Most preinstalled distributions are not open source
Although the AOSP exists, your phone is most likely running a proprietary apps like the GApps or manufacturer specific applications.