The Great Flood was a global cataclysm that covered the whole world, killing all air breathing land animals and birds except those within the ark which Noah had built based on a pattern given him by God Himself. The flood was the Creator's action in response to the level of violence and disinterest that the population of the world had risen.
After a period of intense rainfall and general destruction from below the floors of the oceans, the earth had been covered with water to a depth 15 cubits (about 22.5 feet) above whatever the highest mountain was at the time. This rise took a full 150 days before receding to present levels.
Destruction came from rain and activity under the surface of the seas, and perhaps, the land. After forty days and night the rain stopped but the water rose until all the mountains were covered, the water eventually receded but left much of the Earth deformed and damaged. The Great Flood occurred because of the sinfulness of humanity at the time.