In the most basic terms GOG Galaxy is a competitor to Steam that allows its users to define what exactly they want out of it. GOG Galaxy has been in beta for quite a while now, but with the recent arrival of the massive Update 1.2 it is just about ready for an official release in April.
As for Update 1.2, its biggest and most exciting addition is universal cloud saves. In other words, no matter what game you play, and no matter how old that game may be, you will always have a way to keep your precious save files close at hand. Besides cloud saves the update has also brought with it the ability to customize which features you want used, a new hibernate mode to minimize CPU usage, an improved chat client, a bandwith limiter and scheduler, and FPS counter, and the list goes on for quite a while.
If you would like to learn more about all of the new changes, or just try out GOG Galaxy for the first time, you will find what you seek on the official website. Just keep in mind that Update 1.2 is considered a test update, and as such you will need to opt-in to access all of the new features. If you want the full bug-free experience, however, you will have to wait until April, though the exact release date has not been given just yet.