The tooth you see in your mouth is anchored to the bone with its own root. Every tooth has a different shaped root system. There are nerves and blood vessels traveling through these roots to provide vitality and nourishment to the teeth. If a tooth is either traumatized excessively or exposed to deep decay, the nerve inside the root may "die".
This is sometimes a very painful and debilitating occurrence that requires immediate treatment. If the nerve dies and is not treated the infection in the root will spread to the bone and create swelling, more pain and an inability to chew due to the tooth being actually moved by the pressure from the abscess( infection). Root canal treatment is highly successful and when completed the symptoms will almost always subside. As the tooth does not have blood flowing through it any more, it has a tendency to "dry out". This results in the need for a crown to protect it from cracking in the future.