Lumberyard is designed for cloud-connected, cross-platform games and is "a blend of new and proven technologies from CryEngine, Double Helix, and AWS." There's also of course integration with their Twitch video platform. Both cloud-connected and standalone games can be developed.
It would appear that Lumberyard is in large part based off CryENGINE as last year Amazon and Crytek had done a giant licensing deal around CryENGINE 3 when the German video game company was going through tough financial times. When brought up in the FAQ, "Lumberyard is made up of proven technology from CryEngine, AWS, Twitch, and Double Helix. We’ve hired some of the best game technologists in the world, who have already made over 41 pages of additions, fixes, and improvements to Lumberyard. For example, we’ve integrated a brand new networking layer, GridMate, so your engineers can more easily build low-latency, high-player count, multiplayer games. We’ve also created a new launcher and project configurator so your team can get set up without engineering help. We’ve also created new workflows so your artists can iterate faster and create higher-quality content, including a new particle editor, 2D/UI tool, and cross-platform asset pipeline."
Lumberyard is launching today with support for Microsoft Windows as well as PlayStation 4 and Xbox One. However, Linux and OS X support is being worked on, according to Amazon. Lumberyard is free to download and the full source is available; Amazon's aim is to lure more people to using their AWS Cloud Services and Twitch video platform.
You can learn more about their Lumberyard engine via this aws.amazon.com post. Unfortunately, while the source is publicly available, it's not technically open-source as you are still bound by Amazon's terms and not allowed to make your game open-source.