It's the Windows 10 start menu. It's basically still the mobile-style start screen from Windows 8/2012, but doesn't take up the full screen, and has a vertical

It's the Windows 10 start menu. It's basically still the mobile-style start screen from Windows 8/2012, but doesn't take up the full screen, and has a vertical list of apps on the left-hand side. Classic Shell should continue to work if you'd like to restore sanity.