Semillon is a white grape variety originating in France, and it belongs to the species Vitis vinifera. The grape has a thin skin and produces, in its basic form, a rather neutral, medium-bodied white wine. It is rather the balance between freshness and the development the grape gains over time that makes Semillon interesting. With longer ageing in the bottle, the wines gain complexity, and you will often find notes of lime along the way.
The most important growing areas are Sauternes and Barsac in Bordeaux, as well as Hunter Valley in Australia. In Bordeaux, Semillon is often part of blends, typically together with Sauvignon Blanc, both in dry white wines and in the famous sweet wines. The grape is indeed susceptible to noble rot (botrytis), and it is precisely this fungus that underlies the botrytised, sweet wines from Sauternes and Barsac.
In Australia, Semillon has found a distinctive expression, among other places in Hunter Valley. Here the grape historically appears under synonyms such as Hunter Riesling and Hunter River Riesling, even though it has nothing to do with Riesling. Other well-known synonyms for Semillon are Blanc Doux, Chevier and Chevrier, as well as Colombier and Crucillant.
In terms of style, Semillon thus spans broadly, from dry white wines to sweet wines. The dry versions are typically medium-bodied and, with a few years behind them, can develop a more nuanced character, while the sweet, botrytised wines from Bordeaux are among the most well-known sweet wine styles of all.
You can usefully think of Semillon as a grape that can both stand alone and play a leading role in an assemblage. If you want to explore its range, it makes sense to compare a dry white wine with a sweet, botrytised version and sense just how different the expressions one and the same grape can hold can be.