Android TV, and by association Nexus Player, are the evolution of Chromecast. It essentially eliminates the need to use a middleman device like a phone or tablet if the user chooses, and allows them to interface directly with the device and TV in many cases. While it still retains all the functionality of Chromecast, a dedicated box allows for a much more robust and feature-rich system, as well as more room for future enhancements.
Nexus Player Review: Google Just Brought A Stick To A Gunfight [Updated] was written by the awesome team at Android Police.