Hemorrhoids are essentially varicose veins of the rectum. Strictly speaking there are two types of hemorrhoids, internal and external. Internal hemorrhoids are located inside the rectum and are covered with a relatively fragile lining called the rectal mucosa. Internal hemorrhoids tend to bleed but do not generally cause pain. External hemorrhoids are located outside the rectum and are covered with very sensitive anoderm. These tend to cause pain, burning and itching but do not bleed. Most people however have complex hemorrhoids that originate internally but become so large that they take on characteristics of both internal and external hemorrhoids.
Hemorrhoids are classified based on severity into four grades. First degree hemorrhoids bleed but do not prolapse outside of the anal canal; second degree prolapse outside of the anal canal, usually upon defecation, but retract spontaneously. Third degree hemorrhoids require manual placement back inside of the anal canal after prolapse, and fourth degree hemorrhoids consist of prolapsed tissue that cannot be manually replaced. Fourth degree hemorrhoids are at high risk of becoming strangulated leading to thromboses and severe pain.