After the hors d'oeuvre course is served, the first dish presented in a traditional Japanese meal is most often a course of raw fish or other meat. The general term for this course is o-tsukuri. The root of the word, tsukuru, means to make, create or — if you read into the meaning — to arrange.
Many people are familiar with the term sashimi referring to raw fish. Sashimi literally means "sliced meat." Fish or other meats — chicken, beef, venison, etc. — served raw or very rare and sliced fairly thin, may all be considered sashimi. In turn, the first course, o-tsukuri, may also be referred to as sashimi.
The difference between the words is obtuse, and the one you choose to use is generally a matter of personal preference. However, as a guide, at sushi counters and izakaya, the word sashimi seems most appropriate, while at finer establishments — ryotei and kappo restaurants, for example — the more elegant o-tsukuri term is often employed.