What is applesingle?

Some articles on applesingle:

Apple Single And Apple Double Formats
... AppleSingle Format and AppleDouble Format are file formats developed by Apple Computer to store Mac OS "dual-forked" files on the Unix filesystem being used in A/UX, Apple's first ... AppleSingle combined both file forks and the related Finder meta-file information into a single file, whereas AppleDouble stored them as two separate files ... AppleSingle is similar in concept to the more popular MacBinary format, in that the resource and data forks are combined together with a header containing the Finder information ...