There are a few reasons a tooth could need a root canal. One reason is if a cavity is large enough to expose the tissue inside the tooth. If there is no exposure, a tooth may still need a root canal following treatment due to the inflammation stemming from the proximity of a filling material to the tissue inside the tooth. Another reason could be a tooth that has fractured and exposed the tissue. Occasionally, a traumatic injury with no fracture to a tooth could also be a causative agent for a root canal.
Front teeth typically have one root and the back teeth have three. Root canals are basically filling materials for the canals that travel down the length of the root.