Watch the following videos to make the best use of Lookup Columns in SharePoint 2010.
Lookup column is a very powerful type of column that can be used in any list or library to get information from another list or library on the site. This functionality can be used to centralize and protect your information (for example list of your Customers) and then do a lookup to it from various lists and libraries. This video goes more in depth to show how you can allow multiple values to be selected or enforce unique values in the lookup column.
Lookup columns offer the capability to enforce column relationships and referential integrity. Two types of relationship behavior can be set: Restrict Delete or Cascade Delete. Both of these settings and their impacts are demonstrated in this video.
These videos are available for free until 8/10/2010. They will then be returned to the subscribers only area.