This is the new look for Gmail users courtesy Google.
Most Gmail users won't be impacted by the changes, unless of course they are against change.
If you want to see the new look click on "switch to the new look" that will appear soon on the bottom right of your Gmail screen.
Most of the changes are about customization. If you don't change something yourself, not much will change.
Here's the basic rundown from Google:
Google produced a video to explain the changes.