Microsoft Excel: Functions - VLOOKUP
VLOOKUP is used to find information in another sheet (or file) and display it in your current sheet. For example, you might have a product code and want to know the product description. You would use VLOOKUP to find the product code for you in another location and bring back the description.
To use this effectively you really need to understand Absolute References (using a $ in a formula). Include that module if you aren't sure.