For Mac OS X it's a bit different:

`[mac]` refers to the hardware  
`[osx]` refers to all versions of Mac OS X in general
`[osx-snow-leopard]` and such specify a specific version.

I've put more on the tag info pages: [Mac](http://superuser.com/questions/tagged?tagnames=mac&sort=info&pagesize=50) and [osx](http://superuser.com/questions/tagged?tagnames=osx&sort=info&pagesize=50)