The 27.5" tyre and wheel size now look set to become the standard mountain bike size over the next few years. It's also known as 650B and is, like other sizes, an old size resurrected, and in this case, given a new (and confusing) name. The important dimension is 584 - that's the ETRTO number for this rim and tyre size - it's the diameter in mm of the tyre bead seat in the rim, and is the internal diameter of the tyre.
Moruya Bicycles. ABN 27 559 728 517. 10 Ford St Moruya NSW 2537 Australia.