The Planck density is enormous. It corresponds to the mass of 100 billion galaxies being squeezed into a space the size of an atomic nucleus. If we could extrapolate general relativity all the way back to the big bang the universe would have gone from infinite density to the Planck density in roughly 10-43 seconds (which is, of course, Planck Time (Thanks to Inflation and the Early Universe)
As to Critical density, "...The average density of matter in the universe today that would be needed exactly to halt, at some point in the future, the cosmic expansion. A universe that has precisely the critical density it is said to be flat or Euclidean. If the density of the universe is greater than the critical density, then not only will the expansion be stopped but there will be a collapse of the universe in the distant future..." (Source: Encyclopedia of Science).
By the way, measurements of the density of the observable universe suggest that it does not contain sufficient density to result in the Big Crunch... but will continue to expand indeffinitely...