Chometz refers to food products containing any grain (wheat, barley, rye, oats, or spelt) or grain derivative, not specially prepared for Passover use.
Chometz Gamur, colloquially called "real chometz," refers to products containing fermented grains. These products are biblically prohibited on Passover.
Kitniyos - legumes, are those grains that can be cooked and baked in a fashion similar to chometz grain and yet are not considered, in the eyes of halacha, to be in the same category as chometz. Some examples are rice, corn, peas, mustard seed, and the whole bean family (i.e. kidney, lima, garbanzo, etc.). It is customary for Jews of Ashkenazic descent to refrain from eating kitniyos on Passover.
Kosher for Passover means foods acceptable for use during the Passover holiday which require special preparation. See "chometz above".
Matzoh is specially prepared unleavened bread which is acceptable for Passover use.
Passover, or "Pesach" in Hebrew, is the Jewish holiday commemorating the exodus from Egypt, observed in the spring.
Seder means "order". A seder is the Jewish ritual conducted as part of the observance of Passover. The Haggada, the text from which the seder is conducted, contains the precise order of the prayers, song, discussion, story-telling, eating of ritual foods and the festive meal.