Tropism are movement or bending brought about by unequal growing of two parts of an organism.  It’s a growth  movement in which the direction of movement is dictated by the direction of external stimulus, plant hormones are normally involved in response. E.g phototropism, geotropism. Nastic movement on the other hand, are also growth which takes place in response to external stimulus however, this response is not directional and plants hormones may not be involved rather, environmental factors such as relative humidity, temperature are involved. E.g opening and closing of flower and leaves.
In general, tropism lasts longer than nastic movement.