Packages are a conceptual replacement for the resource fork (see ResEdit), with many advantages: * Resources (strings, images, code, etc) are now stored as discrete files instead of byte ranges within a single file (the document's resource fork). This reduces the need for special resource editors and APIs to access the resource fork -- no more need for ResEdit. * Developers (and users) can manipulate the contents of a package in the Finder by invoking the "Show package contents" contextual menu (right click) command. All Mac OS X applications (and many documents) are packages.