Answer:
When Darwin first published his theories on evolution by natural selection, he envisaged evolution as a slow, steady process, with little variation in the rate of change.
Later discoveries showed that this process was unlikely to be so slow and steady as was thought earlier; that in stead certain conditions enabled lifeforms to change rapidly, to develop significantly different morphologies in the course of less than 10.000 generations even. The first scientists to formulate such a model were S.J. Gould and N. Eldridge, and their model was called 'punctuated equilibrium'.
To summarize: the significant difference between Darwin's gradualism and Gould's punctuated equilibrium is the variation in the rate of change.