ActiveSheet Property [Excel 2003 VBA Language Reference]
Returns an object that represents the active sheet (the sheet on top) in the active workbook or in the specified window or workbook. Returns Nothing if no sheet is active. Read-only.
If you don't specify an object qualifier, this property returns the active sheet in the active workbook.
If a workbook appears in more than one window, the ActiveSheet property may be different in different windows.
This example displays the name of the active sheet.
MsgBox "The name of the active sheet is " & ActiveSheet.Name