I hope this helps. Im from korean familey and we love this stuff .
Nigiri-zushi are bundles of rice, topped with wasabi and fish. Once served, nigiri should be taken by the fingers and dipped fish-side only into soy sauce.
Sashimi is simply raw fish, sliced thinly, served without rice.
Sashimi can consist of salmon, squid, or other fresh seafood.
Martha Stewart's website has a simple recipe for Sashimi Salad made with fresh tuna.
Chirashi-zushi is prepared in a bowl, with several different kinds of fish and vegetables mixed in with rice. It can almost be described as a sushi salad. Check out this helpful step-by-step from East Sea Road on how to prepare Chirashi-zushi
Oshizushi literally means "pressed sushi". It is made by layering the ingredients on top of rice in a wooden mold, known as an Oshizushi-Bako.