What the story with __MACOSX ?

What the story with __MACOSX ?

I've just started writing plugins and noticed that some of the ones I've downloaded have separate versions under directories named "_MACOSX." Obviously this has something to do with Mr. Jobs' company, but I'm confused on two scores. First, why do we need a separate, operating-system-specific version of a plugin? Second, even if we need it, how on earth does the end-user's browser know which one to use since the web page itself generally loads a single plugin via the <script> tag?

Thanks.