What is a look up? I'm fairly new to FM and I was just wondering. Thanks!

It's a feature where Filemaker automatically looks up and enters data from one file into another main file based on some look-up field. Suppose you have a price list file that lists all your stock numbers and corresponding prices. Then, if you have a sales file, you can set it up so that when you enter a stock number in an invoice, it will automatically look up and enter the price from the price list file.

A looked-up value does not automatically update itself (unlike a related value). This feature of lookups can make them ideal when you want to preseve somethings value when it was entered.