Is maize herb or shrub

Wheat (the plant) is, as Cycling-rod says, a cereal grain, which is a type of grass grown for its "fruit". It's definitely not a shrub (a small to medium sized woody plant). It's also not an herb in the culinary sense (a plant grown for its aromatic or savory leaves or flowers), though it is a "herbaceous plant" in the botanical sense (a plant with no persistent woody stem above ground).