Cards are bite-size pieces of information that make up your Playbooks. They are organized on Playbooks in the Playbook Columns. Cards can be on multiple Playbooks or none at all. When a Card is updated, all of the other instances where that Card lives is also updated.
Try to make your cards as succinct as possible!
Keep your Playbooks looking fresh and clean by using the Advanced Editor to create Snippets (Snippets are the preview that you see on a Playbook. When you click into the Card View, you'll see all the other information available!)