Sprint is wireless carrier in the Unites States. Usually jockeying with T-Mobile for position to be the third-largest carrier. Over 54.3 million customers use Sprint's network in the United States, most of them not using Windows Phones.
Over the past few years Sprint hasn't had the strongest offering of Windows Phone devices. As of right now, September 2014, you'll only find the Samsung ATIV S Neo as the only Windows Phone device offered by Sprint. The company recently carried the HTC 8XT, but no longer offers that Windows Phone 8 device.
Before those two handsets, Sprint had failed to release a Windows Phone since the HTC 7 Pro launched in March 2011. Sprint is probably not the best place to be if you're into Windows Phone.