It used to be called a "root canal" until the American Association of Endodontists redesignated it "Non-surgical Endodontic Therapy." It is the same thing, same basic process, same materials.
Instrumentation, irrigation and filling techniques have changed enough to have alleviated a lot of the "experiences" folks used to have with "root canal" treatment so the specialty decided to give the procedure a new name.
The pulp, or soft inner tissue is important during the tooth's development. Once a tooth is fully mature, the tooth can survive without the pulp because the tooth is nourished by tissues surrounding it. The pulp is normally surrounded and protected by a layer of dentin.
When a cavity is very deep a tooth’s pulp (commonly called the “nerve”) can become infected. Rather than extracting the infected tooth root canal therapy can be performed and the tooth can be saved. By removing the infected pulp and sealing the root canal the hard structure of the tooth can be maintained and repaired with a filling or crown.