This article should be retitled Node (Silence in the Library), as a disambiguation from other nodes (Burning Heart etc.).
Talk about it here.
Nodes were machines used as interfaces to the central computer of the Library.
They had limited functionality, answering visitors' simple questions and passing on messages. They looked like an abstract statue with a large headpiece which contained a "real looking flesh aspect", a human face. Such faces were donated by people who had died, a practice apparently quite common in the 51st century. One of the people who had donated a face was Mark Chambers.
Each node had an automated decency filter installed to make sure it remained polite.
Faces were chosen for each user individually from flesh banks by the computer, based on what it thought the user would like.
It was from such a node (with the name 710/aqua) that the Tenth Doctor and Donna got their first clues to what had happened in the Library. It replayed a message by the head librarian, warning visitors to count the shadows.