Sushi is made with white, short-grained, Japanese rice mixed with a dressing made of rice vinegar, sugar, salt, and occasionally kombu and sake. It has to be cooled to room temperature before being used for a filling in a sushi or else it will get too sticky while being seasoned. Traditionally, the mixing is done with a hangiri, which is a round, flat-bottom wooden tub or barrel, and a wooden paddle (shamoji).
Sushi rice (sushi-meshi or su-meshi) is prepared with short-grain Japanese rice, which has a consistency that differs from long-grain strains such as those from India, Thailand, and Vietnam. The essential quality is its stickiness or glutinousness. Rice that is too sticky has a mushy texture; if not sticky enough, it feels dry. Freshly harvested rice (shinmai) typically contains too much water, and requires extra time to drain the rice cooker after washing. In some fusion cuisine restaurants, short grain brown rice and wild rice are also used.
There are regional variations in sushi rice and, of course, individual chefs have their individual methods. Most of the variations are in the rice vinegar dressing: the Kanto region (or East Japan) version of the dressing commonly uses more salt; in Kansai region (or West Japan), the dressing has more sugar.
Source: http://en.wikipedia.org/wiki/Sushi; Sept 28, 2010