See if this definition from the Pelvic Floor & Bladder Glossary Sticky is helpful:
Fistula: An abnormal connection between the bladder and another organ or the skin. <Fistulas usually occur during the healing process after a hysterectomy, infant delivery (vaginal or Cesarean), cancer operations, or radiation therapy. Predisposing conditions for fistula formation includes previous surgery, Diabetes, smoking, Cancer, Radiation or Steroid use. The majority of the fistulas that occur in the United States are due to abdominal surgery (especially abdominal hysterectomies).
Two different types of Fistulas that can occur involving the Bladder are enterovesical and vesicovaginal.
Vesicovaginal Fistula (V.V.F.): An abnormal opening between the bladder (vesico) and the vagina.
Enterovesical Fistula: An abnormal opening or passage between the intestines and bladder.
Rectovaginal Fistula: An Abnormal opening between the rectum and the vagina