Data can be stored in many different formats called 'data types'.
When setting up a database or spreadsheet, it is important that the correct data type is selected for each field. This is because the data type will determine what can be done with the data held in that field e.g. sorting, searching, calculations etc.
It also can determine the format in which data is displayed e.g. a date/time data type will allow you to pick the format of your choice, 14/05/08, 14 May 2008, 14th May 2008 etc.
Finally, some data types are able to automatically validate the data being entered.
Again the date data type provides us with a good example. When you use a date/time data type then each date will be automatically checked to ensure that it can actually exist. For example, you would not be able to enter the date 31/02/2008 - it would automatically be rejected or return an error message.