A field is a column or attribute of information that a user can add to a list.
Each element manifest file that defines site columns must adhere to the Field Definition Schema.
A site column is a reusable column definition, or template, that you can assign to multiple lists across multiple SharePoint sites. Site columns decrease rework and help you ensure consistency of metadata across sites and lists. For example, suppose you define a site column named Customer. Users can add that column to their lists, and reference it in their content types. This ensures that the column has the same attributes—at least to start with—wherever it appears. Much like site content types, you define a site column at the site level, independent of any actual list or content type.
https://msdn.microsoft.com/en-us/library/office/ms196289.aspx
