Deck railing is made up of (1) rails, (2) posts, and (3) infill, as shown below:
Rails are the long, horizontal bars - most railing systems will have a top rail and a bottom rail. Posts are the thick, vertical pieces that give your railing strength. Posts and rails create a square or rectangle; infill sits inside that square or rectangle. Infill can be balusters - smaller, vertical bars parallel to your posts. But railing can also use glass balusters, glass panels, or metal cables as infill.