What's the difference between adding cookbooks to Berksfile using 'cookbook' and adding cookbooks to metadata.rb using 'depends'? For example, if I add to metadata.rb depends 'nginx' do I need t...

According to "berks help update", the command is supposed to: "Update the cookbooks (and dependencies) specified in the Berksfile" (Yes ... that's all it says!). But what exactly does this me...