The basic difference between the plinth beam and tie-beam is that the tie beam can be at any level but the plinth beam is at the plinth level.
What is Plinth Beam?
A plinth beam in a frame structure is meant to join all the columns thereby reducing the effective length and thus reducing the slenderness of the columns.
What is Tie Beam?
A tie beam in structural engineering is like a primary beam that connects two columns at any height above the floor level. Tie beams are mainly provided in roof truss and at plinth level. These beams do not convey any floor loads and only act as length breakers for columns where the floor height is unusually high.
For better understanding watch this video