Marjoram
Sweet marjoram (Origanum majorana) is a tender, delicately flavored relative of oregano, milder and sweeter with a floral, slightly citrusy edge. Pot marjoram (Origanum onites) is a hardier, closer cousin to true oregano, with a stronger, more assertive flavor better suited to robust cooking.
Growing Marjoram
Sweet marjoram is more frost-tender than oregano and is usually grown as an annual outside mild climates, or brought indoors for winter as a potted plant. Pot marjoram is somewhat hardier and behaves more like a true perennial in favorable conditions. Both prefer the same lean, well-drained, sunny conditions as oregano and respond well to the same light, frequent harvesting.
Culinary Uses
Sweet marjoram's delicate flavor is best added toward the end of cooking, in soups, egg dishes, and light sauces where its floral sweetness won't be lost to prolonged heat. Pot marjoram, closer to oregano in strength, holds up to longer cooking and can generally be substituted for oregano in most savory dishes.
If a recipe calls for marjoram and you only have oregano (or the reverse), they can generally substitute for each other — just expect a somewhat stronger result with oregano.