In a multi-worksheet document, you can create calculations in one worksheet that refer to cells in other worksheets, allowing you to perform "three-dimensional" calculations.
To do so, place the name of the desired worksheet before the cell address and separate it with an exclamation point. For example, "Sheet1!B2" refers to cell B2 on worksheet "Sheet1".
An example
Assume you want to create a profit/loss statement. For this purpose, you have created three worksheets named "Sales", "Costs" and "Profit".
Now you want the value in cell D2 of the "Costs" worksheet to be deducted from the value in cell A8 of the "Sales" worksheet and to appear in cell C9 of the "Profit" worksheet.
To do this, switch to the "Profit" worksheet. In it, navigate to cell C9 and enter the following formula:
=Sales!A8 - Costs!D2
After pressing the Enter key↵, cell C9 on the worksheet "Profit" will contain the result of your calculation.