Catheters are flexible tubes inserted into body cavities or vessels to drain fluids, deliver medications, or monitor conditions. Urinary types include indwelling Foley (balloon-retained for continuous drainage), intermittent straight/hydrophilic (self-use), and condom sheath (external for males). Vascular types encompass peripheral IV (<3"), PICC/midline (long-term access), central venous (CVC/tunneled/implanted ports), and arterial lines (hemodynamic monitoring). Specialized variants: suction (airway clearance), dialysis (blood filtration), balloon (angioplasty), and guide (endoscopy). Materials: silicone/PVC/latex/polyurethane; sizes via French scale (3-24 Fr); complications include infection/trauma sterile technique essential.