A root canal treatment is a way to save a dead or infected tooth that would otherwise need to be extracted. A root canal takes out the dead, bacteria-laden remnants of the pulp and replaces it with a filling that extends all the way down the root.
Usually people discover they need a root canal when a tooth suddenly becomes painful because a cavity reaches the pulp space. However, sometimes the only symptom is a chronic, painless drainage around a tooth if it is already dead.
Root canals can only be performed if a tooth is still intact enough to accept a filling. If a tooth is very broken down or split in half, then usually the best option is to remove the tooth and possibly replace it with a partial denture or implant.
After root canal treatment, a tooth isn't as strong as it used to be, so a back tooth that takes significant force typically needs to be strengthened by a crown or an all-surface resin filling to keep it from splitting.