The word sushi actually means seasoned rice - a mix of rice vinegar, sugar, and salt, and making it proper takes sushi chefs years to master. They often mix different varieties of rice, similar to how barristas mixes coffee beans to make their own perfect blend.

The most favored variety of rice for sushi is called Koshihikari, which was first created in 1956, by combining 2 different strains of Nourin no.1 and Nourin no.2. It has become very popular now in Japan, in part due to its good appearance. It is one of the most highly-grown varieties of rice, and its taste is said to differ per region. The starch in the core of the grain is more condensed than other varieties, giving it a firmer and denser texture. This rice also contains aromatic fatty acids that add flavor and moistness.

Hakumai (white rice) generally refers to polished short-grain Japonica rice and is a highly nutritious source of protein, fibre, Vitamin B, calcium and iron. A staple of the Japanese diet since ancient times, hakumai is also an excellent source of energy, and forms part of the traditional Japanese meal. As well as being easier to digest than genmai, hakumai’s glutinous texture means that it is easier to pick up with chopsticks and its mild taste makes it a perfect accompaniment to almost any food.

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Useful links:
Japanese rice
Trevor Corsons arsenal
Rice - Wikipedia