An identification card apparently digitally encoded with a person's personal information. These cards can be scanned (using the correct device), allowing the viewer access to all of said information.
Might be that most folks don't have ident cards -- the only time you ever see them in any of the episodes is when Shepherd Book produces one in Safe. Mal never shows anyone one, just producing (inaccurate) papers on the ship in Safe. (In Bushwhacked he gives his name accurately when asked for it -- at which point the Alliance Captain apparently did a Google search on him. :)
Having an Ident card might be a sign of... more legitimate citizenship; a Starship Troopers sort of arrangement: "Service Guarantees Citizenship."