I have added a document library via VS 2012 project. I noted that document library and document library instance added. whats the difference between two? which one I should add to feature for deploying it. Or I need to add both ones to the feature?
The Library contains the definition of the library itself, including the columns, types, indexes, views, etc., but it does not actually store items. The Instance is what handles that and is typically a single instance of the defined Library and is usually created when the feature that contains it is activated.
If you are a coder, think of the Library as a Class Definition and the Instance as the object created based on the class definition.