Site Column is a reusable column template that be used across different SharePoint sites and lists. Leveraging site columns intelligently improves data consistency and simplifies data entry. I have written a quick tutorial named How to Create a Site Column in MOSS 2007 & WSS 3.0 on eHow.
1. Click on “Site Actions” > “Site Settings”.
2. Under Galleries, click “Site columns”.
3. Select the desired data type of the column.
4. If you have not created a new group for the custom site columns, it is highly recommended that you do so now. Otherwise, select “Existing Group” and pick a group.
5. In the Additional Column Settings section, select as many as additional column settings as you like. The options available in this section depend on the column type that you selected in Step 3.
6. Click “OK”
