So you have a very deep cavity (decay) that needs more than just a filling to fix. The cavity is so deep it has reached the nerve and blood supply center of the tooth (pulp) and also has reached the root surfaces. Your dentist tells you there is good news and bad news. The good news is you can save the tooth. The bad news is it is going to require a root canal, crown lengthening surgery, a core buildup and a crown to fix it. Not only are you confused by what these treatment options mean or attempt to accomplish,  but they come with a considerable amount of investment of your time and money to correct. It can get rather expensive to save a tooth when the decay has advanced to such a severe state.