Toothache is the common term for pulpitis, or inflammation of the tooth pulp. Tooth pulp is the living tissue found in ther central portion of the tooth and it is a fragile tissue that contains blood vessels, nerves and connective tissue. The pulp is protected by the encasing hard tooth structure from bacterial infection.  In the presence of decay, fracture, cracks or gum disease, the pulp tissue will be inflamed resulting in pulpitis/toothache.  If pulpitis is allowed to progress, the bacterial infection will cause the pulp to degenerate and there will be an infected space within the tooth and the inflammation could also spread out into the bone tissue around the root of the tooth.