What is the difference between a root canal and pulpal debridement?

Look it up. You need to see the basic parts of what makes up an entire tooth and all adjacent structures. Too detailed for answer here.
Several differences. Pulpal debridement is simply the removal of the pulp/nerve from a tooth. This is one of the first steps of a root canal and is often done to provide pain relief. A root canal involves pulpal debridement in addition to several more steps to disinfect, shape, and seal the canal space. A tooth that has had pulpal debridement must be completed with a full root canal.
They go together. A pulpal debridement may be used when the tooth is initially opened up for pain control reasons in preparation for later having a root canal done in a permanent tooth. Root canal refers to the completed process of debridement and obturation (filling up) of the pulp spaces. In a baby tooth pulpal debride is done if much decay and prior to a silver crown.