Cards are the basic unit of a board, within the Trello web app.
Intro from Trello website:
Cards are the basic unit of a board. They might represent new software features, story leads, legal cases, things to buy at the store, freelance clients, potential employees, or customer support issues. Drag cards from list to list to indicate progression, status, or what-have-you. You can add people to cards, label them, vote, attach files, start conversations, create task checklists, and so on.
Find a more detailed functionallity at the trello website: https://trello.com/guide.