Kosher means proper as related to kashrut, or Jewish dietary law. Based on scripture and rabbinic scholarship, the laws are extremely complex, but here are the fundamentals.
Step 1: Eat permitted meats Eat only mammals that chew their cud and are cloven-hoofed. Accepted meats include beef, venison, goat, and lamb or mutton.
Step 2: Eat permitted poultry Among poultry, chicken, turkey, duck, and goose are permitted.
Step 3: Refrain from eating all shellfish Refrain from eating all shellfish, such as clams, mussels, shrimp, lobster, oysters, and crabs.
Step 4: Eat fish w/ fins & scales Eat only fish with fins and scales.
Step 5: Look for kosher certification Meat and poultry must be slaughtered in the prescribed manner. Look for kosher certification at your butcher or supermarket.
TIP: On packaging, look for trademarked emblems of kosher certification organizations, like a 'U' inside a circle, a 'K' inside a circle, or a 'K' inside a star. 'P' means kosher for Passover.
Step 6: Cook meat & dairy separately Cook meat and dairy using separate utensils and pots.
Step 7: Eat meat & dairy separately Do not eat meat with any dairy products. Use separate plates for the two foods.
Step 8: Wait 6 hours between eating meat & dairy In general wait 6 hours after eating meat to consume any dairy products.
Step 9: Buy kosher eggs & milk Buy eggs from kosher birds and milk from kosher mammals.
Step 10: Check fruit for worms or bugs All fruits and vegetables are kosher, but make sure there are no worms or bugs in or on them.
Step 11: Designate one sink for meat & one for dairy If possible, designate one sink for meat and the other for dairy.
Step 12: If using one sink ... If using one sink, wash utensils, pans, and plates in separate dishpans and dry dishes on separate racks. Do not let the utensils, pots, plates, or racks touch the sink. Do not soak dishes or pots directly in a sink used for both meat and dairy.
Step 13: Make sure grape products are kosher Wine and other grape products, like grape jelly, must be kosher.
FACT: In the U.S.—as in Israel—the lickable glue on postage stamps is not only kosher, but vegetarian.