A dark spot on a tooth or slight sensitivity are not always the result of decay, but when decay is found, a filling may be necessary. Decay can be detected with an x-ray, or by probing with a dental explorer (decayed enamel is softer and sticky).
Restoring the tooth involves removing the decay and placing a filling. For most patients, a local anesthetic will be necessary for this type of treatment. After a filling is done, some sensitivity, especially with large fillings, is normal. This sensitivity should only decrease over time.
This video outlines the steps involved in filling a tooth:
If caught early, decay can be fixed easily with a filling. If it left unattended, decay will spread deeper into the dentin of a tooth, and eventually into the inner pulp chamber, causing the tooth to become infected. Once this happens, much more extensive (and expensive) dental work will be required in order to restore the tooth. Routine dental cleanings are a simple and cost effective way to prevent or catch tooth decay before this happens.