Site columns and Content Type in SharePoint 2010
A site columns can be created on the root site collection level and it would be accessible in all site, sub site and list. Site Columns provide the filed types and describe their metadata so that it can be shared at the site collection level. Site Columns provided the reusability concepts of columns. It’s also helpful in search query and improved the quality of the search result. You can also create the lookup column facility using the site columns in list.
A site column can be created from site action menu->Site settings ->Gallery->Site Columns.
Content Types are basically a way of grouping Site Columns together into a reusable group that represents some kind of content to us.