Food is considered kosher if it is prepared in accordance with Jewish dietary laws and, if not cooked at home in a kosher kitchen, prepared under the supervision of a rabbi. Jewish dietary laws are many and varied; the most familiar are those forbidding consumption of pork and shellfish and the combining of meat and dairy. Whether traditional or otherwise, all foods eaten by observant Jews must follow these laws.
Beef and lamb are kosher if slaughtered humanely in accordance with kosher law (also known as kashrut), as are goats and deer. Chicken, duck, goose and turkey are also kosher.
Jewish dietary law permits eating fish with fins and scales--which means salmon, tuna, carp and herring are permitted, but catfish are not. Also, they do not consider fish to be meat and therefore fish products may be consumed with dairy (lox with cream cheese on a bagel is allowed).
Eggs, like fish, are considered pareve or neutral (neither flesh nor dairy) and can appear in any kosher dish.
All vegan food is considered kosher; there are no forbidden plant-based foods in Jewish law. Vegetarian food incorporating dairy is kosher as long as the milk comes from a kosher animal such as a cow, sheep, goat or other cloven-hooved animal.