Site Columns
Once you’ve chosen a column type, you can create that column again and again, on as many Lists and Libraries as you like. But each time you do that, you run the risk of introducing a typo or getting a setting wrong.
Cut down on the typing and the typos by using Site Columns instead. You can create a Site Column once, and reuse it on as many Lists and Libraries as you like. Once created, you can use your Site Column within the current site and below. For maximum reuse, create Site Columns at the root web or top-level site in your Site Collection.
When naming your Site Column, don’t put spaces in the title. The internal name of a Site Column has spaces replaced with the ungainly string “_x0020_”. This causes developers lots of headaches later. By all means rename/retitle your Site Column with proper spaces, if you want to, after you’ve created it.
Site Columns help with consistency, but for the biggest return on investment, use them with Content Types.