The Microsoft® Office help website for Access defines a lookup column as follows: “A lookup column (or field) is a field in a table whose value is retrieved from another table or from a value list. You can use a lookup column to display a list of choices in a combo box or list box. […]
Use Site Columns to Ensure Uniformity of Terms for Useful Management of Taxonomies in SharePoint Server 2010
In order to build a useful search feature for SharePoint Server 2010, keywords must be consistently used across a SharePoint site and sub sites. Site Columns provide us with a means of enforcing some uniformity over the method by which text information is organized into terms and groups of related text information into term sets. […]