“Card, Conversation, Confirmation”. This formula by Ron Jeffries  captures the components of a well-formed Product Backlog item or User Story.
A “Card” (or often a Post-It note): a physical token giving tangible and durable form to what would otherwise only be an abstraction.
If the Product Backlog item fits on a single index card then it can probably reach Done by the end of a single Sprint.
A “conversation” taking place at different time and places during a project between the various people concerned by a given feature of a software product: customers, users, developers, testers; this conversation is largely verbal but most often supplemented by documentation.
The conversation addresses questions like:
The “confirmation”: consensus by the team that the objectives and the scope of the User Story is understood.
1. 2001: the Card, Conversation, Confirmation model was proposed by Ron Jeffries to distinguish “social” User Stories from “documentary” requirements practices such as Use Cases.