Calculated columns are the SharePoint equivalent of Excel formulae. They allow you display values that are calculated from the values of one or more other columns. These calculations can be made using standard mathematical operators and SharePoint's built-in functions.
The MSDN guide to calculated columns: http://msdn.microsoft.com/en-us/library/bb862071.aspx
The full function reference: http://office.microsoft.com/en-us/windows-sharepoint-services-help/CH001171117.aspx