A foley catheter is a thin, sterile tube inserted into the bladder to drain urine. Because it can be left in place in the bladder for a period of time, it is also called an indwelling catheter. It is held in place with a balloon at the end, which is filled with sterile water to prevent the catheter from being removed from the bladder. The urine drains through the catheter tube into a bag, which is emptied when full. The procedure to insert a catheter is called catheterization.