On their own, websites, Excel spreadsheets and the like are very useful. They can provide an infinite range of ways of messing with data. However, what if you wanted to re-use specific 'ways of messing with data'? Excel provides a system of Data Queries, whereby your spreadsheet can connect to other places in order to request data or calculations. The mechanism by which the provider enables Excel to request data is called an API - an interface for an application. The user doesn't need to know anything about the API other than what data or function is wanted.