Semillon is a thin skinned non-aromatic white variety with medium to high acidity.
When produced as a dry wine, it tends to show subtle flavours of lemon and apple with occasional grassy notes if the grapes are picked early. In it's youth, it can be quite a neutral looking wine and winemakers often choose to build flavour and texture into it by using oak and lees work, or partial skin contact.
Due to the grape's thin skins, it is prone to botrytis if grown in the right conditions. It is the workhorse in Sauternes, where it provides the honey and marmalade notes and also helps to soften Sauvignon Blanc's intensity. It is often the star in Australian dessert wines too.
Hunter Valley Semillons are famed for their longevity, and indeed can take a decade or more to show the much sought after tertiary characters of honey, beeswax, lanolin, toast and hay. By and large, these examples are unoaked.
Winemakers in other Australian regions tend to follow the Bordelaise example, by harvesting later and rounding out fruit with oak influence.
In terms of food pairing, Semillon's neutrality makes it a versatile and easy companion. Pair young, lean wines with vegetables and salads and pair older or richer examples with roast white meat.