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Seriously, nobody knows enough to do a basic rewrite?
This definition clearly comes from the Kikkoman people. It has a LOT of problems. Somebody really should rewrite. I have written an entry on sashimi, but I am no expert on sushi. What I do know is that sushi is basically vinegared, seasoned rice, usually combined with something else -- fish, pickle, vegetable, egg, whatever. In addition to the styles already listed by colombochute, there is also chirashizushi (stuff scattered on top of sushi rice), Kansai-style mackerel sushi (mackerel wrapped around strong sushi rice and preserved briefly), pressed sushi, preserved sushi, and on and on. I would fix this, but I'm not sufficiently knowledgeable.
Sushi covers things as different as nigiri(sushi), maki(sushi), and temaki(sushi). The forming of these different sushis uses very different techniques. The definition above should be reviewed as it makes a confusion between nigiri and maki.
Besides, I never heard about ginger being used in sushi, but I might be wrong on this one.
Sashimi does not require the fish to be set on a mound of rice, nor does it require marinating in sake.