Think of the tooth as having a tube or channel running from the top of the tooth to the tip of the root. This tube or channel is called the root canal system of the tooth.
Some teeth have more than one channel and some have channels going sideways.
When a tooth is abscessed or infected, it is the pulp or channel that is infected with bacteria or dieing from some sort of injury. Root canal therapy involves opening into the top of the tooth, removing the infected soft pulp material, shaping and cleaning the channel with very small instruments, and sealing the channel with the appropriate material to keep bacteria from getting inside again. Usually, after root canal therapy a crown will be placed over the tooth to protect it. Gum disease usually means your gums have detached from the root of your teeth due to plaque and tarter build up. The build up gets under your gums and releases a toxic chemical from its bacteria.