OS X is a UNIX, so file extensions don't matter a whole lot. You can double click a plain Mach-O executable file and it'll run it in a Terminal window. For application bundles (folder with name ending in .app), OS X will warn you if the file was downloaded/not signed by a key trusted by Apple.
While it's true that they don't have meaning to the lower levels of the system, file extensions matter a whole lot to the higher-level frameworks and the GUI that ordinary people use.
Thanks, as someone who hasn't used a UNIX os since Highschool I am not very familiar with them.
As an aside I am thinking about installing a light UNIX distro on my netbook. Win 7 is so painful with only 1GB of RAM. (I love being poor student).
Now I got some money so I can either buy a new laptop for $350 or attempt to save some money by installing a light UNIX distro. Problem is, I dont want to learn another OS.
Is this the case for Apple's OS well (Yosemite or whatever its called nowadays)?