Flood Plain: This is the area beside a river in its lower course. As meanders move down the course of the river, they erode and widen the previously v-shaped valley and the river deposits material in the valley when it floods. Over time this fine material builds up to create a flood plain.
Levees: These are natural embankments/walls found along the sides of a river channel in the lower course. During a flood, the river deposits the heaviest material first, nearer the channel, which builds up into these banks over time.
So the difference is flood plains are built up with fine material whilst levees are built up with heavy, coarse material.
Hope that helps but if you still don't understand, look at this website, as it has some really helpful diagrams.