To Quote the wiki:
2.1.1 Published Topics
~<name>/global_plan (nav_msgs/Path)
The portion of the global plan that
the local planner is currently
attempting to follow. Used primarily
for visualization purposes.
~<name>/local_plan (nav_msgs/Path) The
local plan or trajectory that scored
the highest on the last cycle. Used
primarily for visualization purposes.
Basically, global_plan
is what is currently viewed as "follow this path", i.e. the Input to the planner, which is not the full path to the goal. Otherwise, it might be that tremendous shortcuts could appear. How long this is depends on several Parameters, maily those in 2.2.3 of the wiki.
The local plan is the adjusted/sampled trajectory which is executed in the following, i.e. the Output. The execution, however, happens in terms of velocities that are published through move_base.