About five years ago, the design of most Google apps started converging towards a common design language, which was eventually revealed to the developer community during Google's I/O 2014 conference. I still remember seeing this short video for the first time, thinking how all of it made so much sense:
The design language Google's designers came up with was called Material Design, referencing the idea of making a user interface behave in a physical way, with animations and drop shadows not only there because they "look cool", but because depth and a sense of "what went where" are useful for users of an app to understand what happens on the screen. Here's another video, using some of the same animations but also containing explanations from the people who created Material Design:
The Material Design language is now used by all Google apps, but is also available to app and web developers everywhere. The variant currently used by Google itself has, in my opinion, too much white and not enough contrast - but that has always been a Google problem. ;)
Other than that, I think it is a great tool for every developer to use, freeing them from some of the burden that typically comes with designing an app from scratch. I love Material Design for that reason, although I'm sometimes still hitting roadblocks when it comes to implementing one or another detail. With Google+ going away, where I found a good community of people to discuss these things with, perhaps this conversation can be continued on Cake.