Android phones represent one of the fastest-growing segments of the mobile phone market. Fueled by an open source operating system created by Google, Android phones have become ubiquitous, collectively surpassing the Apple iPhone in popularity. Although other smart phone operating systems exist, including Microsoft Windows Phone 7 and the BlackBerry operating system, the popularity of Android is rivaled primarily by Apple's iOS The best way to find an Android phone is to visit a wireless network store either in person or online. Customers who already have mobile phone service may want to begin shopping with their current provider and then compare what other networks offer. The important thing to remember is that customers who change networks must often pay early termination charges if they are not at the end of their contract. Android phones come in a number of brands. Among the most popular are Samsung, HTC and Motorola, although other brands such as Sony and LG are also available. Different wireless networks often have similar brands, but they usually have different models, so comparing available phones from each one is important. Wireless customers can often save money on Android phones when they buy them used online. When looking for a used phone, shoppers must make sure that the phone will work on their network. If any doubt exists, buyers should contact their wireless provider and supply them with the brand and model number of the used phone to see if they will support its use. Android phones work a lot like Apple iPhones, meaning that they rely primarily on a touchscreen for input rather than a keyboard or a mouse. They usually include internet connections that allow users to stay connected on Facebook and Twitter, download music , surf the web and check email. Thousands of applications that run on Android phones are available either for free or for a price from the Android Market. Games, reference materials, camera utilities and productivity apps are among the software users can use.