Lightning Base Components introduced in winter 17 release is documented here .Please note that many of these components are in BETA mode .
One of the things as a Visualforce developer I found harder when I was new to development was documentation did poor job to help imagine the UI and how the components will render .I see the same issue with lightning components developer guide .
While the base components are listed and sample code is shown ,still to my fellow designers I am sure this is night mare to imagine or even for developers there will need to copy paste the code and preview how the markup renders which is definitely not respecting their time .
I quickly made a small documentation that would help the designers as well .
Here is the link to the documentation on git
Here is a small unmanaged package in case you want to play around with all components
The package contains all components inside a simple lightning App Page