ID Cards are visual representations on the supercomputer display. They allow Jeremie (or whoever is operating the computer) to see who or what is on Lyoko and how many lifepoints they have. On the front of the card is a profile picture of the person or thing the card represents, their lifepoints and their attack points. On the back is a symbol that represents the possessor of the card.
When a person is being virtualized, the operator selects the ID card for the corresponding person from a set of five. During the scanning process, the card's colors brighten to show that they are prepared to be sent to Lyoko. When they are devirtualized, the card's colors dull from the top to bottom before vanishing, leaving a grey-colored card. It is believed that while the card stays grey, the person it represents cannot be re-virtualized.
When William was devirtualized in Down to Earth and successfully brought back to Earth, the colors of his card returned from bottom to top. When all the colors were restored, the card glowed once, and William came out of a scanner.
In Season 5, ID cards now show if there are any malfunctions in someone's avatar and their code. ID cards also make it easier for someone to know when they can be re-virtualized, as the person's ID picture glows red after devirtualization.