Maize is a member of the monocots, and broad bean is a dicot plant. There are many structural differences between moncots and dicots, including critical differences in the seeds.
The most obvious difference, and the one for which the groups are named, is the number of cotyledons. Monocots have one cotyledon, or seed leaf, which dicots have two. Additionally, the cotyledons themselves are quite different between the two groups.