A fistula is an abnormal connection or passageway that forms between two organs or vessels in the body, often resulting from injury, surgery, infection, or inflammation. Common types include anal fistulas, which connect the anal canal to the skin, and arteriovenous fistulas, which link arteries and veins. Fistulas can lead to various complications, including infection and fluid leakage, and may require medical treatment or surgical intervention to close them.