The grapes used in the production of Rutherglen are Brown Muscat (a clone of the old world Muscat Blanc a Petits Grains which is found in the classic French Vins Doux Naturels) and so-called "Tokay" (Muscadelle).
Viticulturally, Rutherglen is unirrigated (water being scarce and so strictly controlled by the Australian authorities). The grapes shrivel, concentrating the sugar and allowing the grapes to achieve 20-22% potential abv without botrytis.
The wine is fortified to interrupt fermentation, in the traditional Port manner, leaving a minimum 160g/l sugar. Ageing takes place in a solera-like system. It is oxidative and evaporation takes place. Once bottled, Rutherglen does not develop further.
The 3 styles are Classic (which must be 5 years old); Grand (10 yo); and Rare (15 yo, but is usually much older). These categories also indicate increasing quality, concentration and sweetness.
(Sources: WSET materials, The Oxford Companion to Wine)
