Originally a company that made software for mobile phones, Android was bought by Google in July 2005. As soon as the news reached everyone, rumors spread that Google was planning on entering the mobile market, but their intentions and plans were unknown. Finally, in September 2007, those rumors were confirmed when it was discovered that Google had applied for various patents related to mobile phones.
A beast the size of Google was always going to attract a lot of eyeballs. And now that it had decided to enter the mobile-phone market, lots of other companies wanted a piece of the pie, too. The result of this was the Open Handset Alliance, founded in November 2007. The alliance is made up of the following companies: Google, HTC, Intel, Motorola, Qualcomm, T-Mobile, Sprint Nextel, NVIDIA, Sony Ericsson, Vodafone, ARM, Asus, Toshiba, Garmin, and a few more.
The Android operating system was revealed to be an open-source distribution of Linux, which would be available on a variety of devices from various manufacturers like HTC, Samsung, Lenovo, Sony Ericsson, Acer and many more. Owing to the modularity of the kernel and its open-source nature, Android is expected to be ported to hundreds of devices with varied feature sets. For more info, check out the entry for Android on Wikipedia.