As a free, open-source operating system built upon Linux, Android has a large number of variations, either as a result of modifications made by the Open Handset Alliance or hardware manufacturers.
Although the Open Handset Alliance is led by Google, not all Android devices will have Google services included, as they are added as licensed products that are not free for manufacturers.
Google services include Gmail, Google Maps, Google Navigation and the Android Market.
Devices without Google's Android Market are typically built with alternatives for users to obtain apps for their device.
Some Android-based devices use the operating system as a foundation for their devices' software, which may result in a user interface that does not resemble that of the base Android OS. Notable devices with this characteristic are the Barnes and Noble NOOK Color and the Amazon Kindle Fire.
