Let's say you want to determine whether a salesperson sold more this year over last year so that you can calculate an appropriate bonus, or one warehouse contains more of an item than another warehouse in order to keep inventory levels balanced. To check if a number is greater than or less than another number, use the IF function.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How to copy an example
 Create a blank workbook or worksheet.
 Select the example in the Help topic.
Note Do not select the row or column headers.
Selecting an example from Help
 Press CTRL+C.
 In the worksheet, select cell A1, and press CTRL+V.
 To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

A 
Data 
15,000 
9,000 
8,000 
Formula 
Description (Result) 
=A2>A3 
Is A2 greater than number in A3? (TRUE) 
=IF(A3<=A4, "OK","Not OK") 
Is A3 less than or equal to the number in A4? (Not OK) 

Function details
IF