Hemorrhoids are basically large blood vessels within the rectum. There are two different of hemorrhoids -- one, internal hemorrhoids, and one other, external hemorrhoids. The external hemorrhoids are the kind of hemorrhoids that actually hang outside the anus. The internal hemorrhoids are the ones that are inside the anus. Both of them can cause a lot of pain, and both of them can cause bleeding. Most common cause of hemorrhoids and hemorrhoidal pain is constipation. Basically, hemorrhoids are a cluster of blood vessels within the rectum, and their main function is, basically, to keep the anus closed, so that you don't spill any stool out, and basically, keep the contents within the rectum encased. People who've chronic constipation can intermittently engorge those blood vessels, and if they strain, they can tear those blood vessels and cause bleeding, that's what a lot of people will complain about when they have hemorrhoids, hemorrhoidal pain or hemorrhoidal bleeding.