Answer Description

Finder is the GUI based file and folder manager in Mac OSX. Nautilus is a file and folder management application in Linux. Explorer is for Windows.

Wikipedia

The Finder is the default file manager and graphical user interface shell used on all Macintosh operating systems. Described in its "About" window as "The Macintosh Desktop Experience", it is responsible for the launching of other applications, and for the overall user management of files, disks, and network volumes. It was introduced with the first Macintosh computer, and also exists as part of GS/OS on the Apple IIGS. It was rewritten completely with the release of Mac OS X in 2001. In a tradition dating back to the Classic Mac OS of the 1980s and 1990s, the Finder icon is the smiling screen of a computer, known as the Happy Mac logo.