Both sashimi and sushi are Japanese fish dishes. Confuse one with the other at your own risk. Sushi is slices of raw fish, mixed with, laid over, or wrapped in rice prepared with vinegar. Variants include California sushi roll, often served with cucumber, avocado and mango cores-vegetarian sushi!
Sashimi is very fresh and very raw fish, often served in thin slices half an inch or a quarter of an inch thick. Both sushi and sashimi are accompanied by soy sauce and the very pungent wasabi paste.
Thin slices of pickled ginger are served alongside to clean your palate between each bite, especially if that extra wasabi "shoots through your nose". And, yes, learn to use chopsticks. Forks and knives are not impolite, but may not be available at Japanese restaurants.