On a normal worksheet, you can refer to other cells using one of these methods:
A cell on this sheet:
A1
A cell on a specific sheet:
Sheet2!A1
A cell on a specific sheet in a specific file:
[File2.xls]Sheet3!A1
On a macro sheet, there is an additional method to refer to other cells, like this:
!A1
this means a cell on whichever sheet is the currently "active" (visible) sheet.