A guidewire is a thin wire used to navigate vessels or tracts, while a catheter is a tube advanced over the guidewire to deliver fluids, drain, or provide access. They are often used together.

Catheter size is measured in French (Fr) units, where a higher number means a larger outer diameter. The size is selected to match the vessel or tract and the intended use.

Guidewires provide a track to safely advance catheters and other devices to a target site, helping navigate the vascular system or other anatomical pathways.