so whats the significance of these terms, force user and force sensitive?

Does it mean like cards that say "FORCE" ect can only be played if you have a force user or a force sensitive unit in play?

Nope. Those are traits. They mean nothing unless another card references them. (See Return of the Jedi, or the lightsabers, for example.)

My guess is that it opens the door for FF to create cards that only affect or enhance CHARACTER units that are "Force Sensitive" or "Force User". Thematically, Force User is only used to describe Jedi or Sith characters and Force Sensitive is used for characters still training to become a Jedi or Sith Lord.

If we are lucky, in future expansion sets we will see a Force User version of Leia.

I just realized that "Return of the Jedi" only applies to Force Users, not Force Sensitive character units, so the difference does matter.