When out on the map screen if you mouse over hexes, or in the strategic view you can see icons if a hex contains a resource. Wheat and Bananas show up when you do this, yet they are not luxury/strategic resources. So why do they show up?
They are food resources. They are similar to luxury/strategic resources in that they give you an additional bonus to the output of the hex if you build their corresponding building on the hex, but unlike luxury/strategic resources they don't provide any bonus other than to the output of the hex, and so they cannot be traded.
Other examples are Fish, Deer, and Sheep.