Developer
In this mode, all the files in pub/static/ are symlinks to the original file. Exceptions are thrown and errors are displayed in the front end. This mode makes pages load very slowly, but makes it easier to debug, as it compiles and loads static files every time. Cache can still be enabled.
Default
This default is enabled out-of-the-box. It is a state in between production and developer, as the files are generated when they are needed. I.e. CSS files are generated using several LESS files in several locations. These files will be generated only when they are needed by the front end, and will not be generated again the next time they are needed.
Production
This mode should be enabled for all Magento 2 websites in production, as all the required files are generated and placed in the pub/static folder.