A "fissure" is an elongated ulcer in the long axis of the lower anal canal. "Fissure-in-Ano"(the medical term) is a disorder when a fissure or ulcer occurs on the anal verge. This causes excruciating pain and/or bleeding while passing motion. The pain may last for few minutes or may persist for few hours or may be present throughout the day.
You can see how a Fissure grows with this animation. Further below is a list of pages that will help you understand Fissure better.