A floodplain is an area of land which is covered in water when the river bursts its banks.
A floodplain forms due to both erosion and deposition. Erosion by meanders removes any interlocking spurs, creating a wide, flat area on either side of the river. During a flood, material being carried by the river is deposited (as the river loses its speed and energy to transport it). Over time, the height of the floodplain increases as material is deposited on either side of the river.
Levees are elevated banks of deposited material at the sides of the river that stand above the level of the floodplain.