Root canal therapy involves the process of removing the pulp from an infected tooth and replacing it with a biocompatible material called gutta-percha. Gutta percha does not react within the human body, making it an excellent material for root canals.
The most common cause for needing a root canal is an infection. Once the root is infected, antibiotics will help, but they cannot clear the source of the infection. At that point the only long-term solutions are to extract the tooth or perform root canal therapy. Sometimes the tooth is unrestorable, but generally we can save the tooth through root canal therapy.
Teeth that have undergone root canal therapy are more prone to breaking and fracturing and therefore generally need to be crowned. This protects the tooth long term.