Sushi / sashimi knives
The traditional knife for sushi and sashimi is the yanagi sashimi.  It features a long blade (approx 10in or 270mm) that has a chisel ground blade which is often hollow ground on the back, called urasaki.  The long blade allows you to cut thin slices in one continuous motion so that you don't you don't have slashes marks from changing direction.  There are also other traditional Japanese knives for specific food preparation tasks such as vegetable chopping and meat preparation.  If you already have European-style knives you are comfortable with, there is no need to purchase Japanese-style knives for those tasks.