Hemorrhoids are annular vascular structures composed of arterio-venous blood vessels and connective tissue that are located underneath the rectal mucosa. Physiologically they contribute to the anal closure.
In common parlance "hemorrhoids" does not refer to the physiological anal cushions, but to an pathological enlargment of those structures, mostly caused by dilated or thrombosed veins in hemorrhoidal plexus, causing symptoms such as pain or bleeding.
Pictured are hemorrhoids grade IV, equivalent to an anal prolapse.