It's an overflow of water onto land that's normally dry. Sometimes it's just a few inches and other times it's several feet. It can occur quickly or gradually over many weeks.
Normal homeowner policies don't cover flood damage. It won't cover damage by excessive rain, a broken dam, or hurricanes. Flood insurance is a standalone policy.
You have two options: private insurance companies or the National Flood Insurance Program. Call a few places to get the best prices for the best plan.
It depends. The average cost is around $700 but your rate may be higher or lower. Get multiple estimates before purchasing your plan.
Sometimes your lender will require you to have flood insurance. Sometimes the law will require it. Check with your lender policies, but also consider the area you live and the value of being protected.