The Android operating system (OS) is based on the open Linux kernel. Unlike the iPhone OS, Android is open source, meaning developers can modify and customize the OS for each phone. Therefore, different Android-based phones may have different graphical user interfaces GUIs even though they use the same OS.
Android phones typically come with several built-in applications and also support third-party programs. Developers can create programs for Android using the free Android SDK (Software Developer Kit). Android programs are written in Java and run through Google's "Davlik" virtual machine, which is optimized for mobile devices. Users can download Android "apps" from the online Android Market.
Since several manufacturers make Android-based phones, it is not always easy to tell if a phone is running the Android operating system. If you are unsure what operating system a phone uses, you can often find the system information by selecting "About" in the Settings menu. The name "Android" comes from the term android, which refers to a robot designed to look and act like a human.