There are a number of different types of water pump, but at a basic level, water pumps, as a general rule, function by manipulating pressure. This means a water pump can raise water from a low level to a high level. (See image below)
Three Water Pumps In Action
Water will always move from an area of high pressure to an area of low pressure so a pump works by creating an area of low pressure and helping to lift or propel the water with help from atmospheric pressure.
This makes powered water pumps very useful when to comes to emptying ponds, flooded basements or pools.