Hametz is flour (defined as from wheat, oatmeal, spelt, rye or barley) mixed with water for longer than 18 minutes (from the moment the water touches the flour until it is baked). So this includes cake, crackers, noodles, including the oatmeal or flour used to thicken soup, hot cereals, beer, whiskey, alcohol from the grains mentioned above even in non-edible items (for instance perhaps in hand wipes); many, many dishes. Not one drop of this mixture should be eaten, cooked, prepared, or owned by a Jew during the days of Passover. (It can be sold and locked away in your vicinity).
There is a minhag (custom) for Ashkenazi Jews and a few different Sephardim not to eat kitniyot which is grains or legumes that could be refined that are not Hametz but look like it; rice, corn, beans, peanuts, etc. What are not in this list are nuts, fruits, quinoa, and more – if you want to follow this minhag it is best to look at Aish or speak to an Orthodox Rabbi.
There is still another minhag to not mix matzah – bread prepared with only flour and water and baked in less than 18 minutes – with anything wet; “garbrochs”. This is not considered Hametz (matzah balls, cakes from matzah meal) but many people hold this custom dearly.
And still another custom (in Pesach – there are many) of the flour to be used in the matzahs need to be guarded from the moment they are taken off the wheat stalk until they are made into matzah, Shmuriah.
There are many more customs but not to be confused with the mitzvah which is only HAMETZ (from the first paragraph). This is the essential.