A worksheet function is a function or formula that is used within the spreadsheet of a spreadsheet program like excel or openoffice-calc. The function can be a user-defined function via vba or a program-ready function like
=vlookup(). These functions are used to perform simple or complex calculations with the data from the spreadsheet.
Examples of pre-defined functions for Excel 2003 may be found here.
In Excel further functions may be created at will. These are termed User Defined Functions or UDFs.