Surface water runoff is a step in the water cycle on Earth. When precipitation occurs, water only has a few locations where it can go. Water can infiltrate into the ground, evaporate, or become runoff.
Runoff is the short way of saying surface water runoff. Water that does not get absorbed into the soil, or rise back into the atmosphere as water vapor, will run off surfaces collecting in varied locations. (In low-lying areas, on floodplains, etc.) The environment in which water from precipitation lands will determine the likelihood of surface runoff. For instance, paved areas prevent water from infiltrating into the ground. The water will run off the surface if evaporation does not take place.
Flooding can occur if the amount of precipitation in an area exceeds the evaporation rate and infiltration capacity of the soil. Significant floods can also occur as water hits paved areas and has not chance to infiltrate into the ground. Hard ground surfaces and impermeable clay surfaces will also prevent water from infiltrating and can cause flash floods.