Rachel Andrew has a nice grid example site with a page that explains the differences.
auto-fill vs. auto-fit
In the event of an overflow
auto-fill will wrap to a new line too.
According to the specs...
The auto-fit keyword behaves the same as auto-fill, except that after grid item placement any empty repeated tracks are collapsed. An empty track is one with no in-flow grid items placed into or spanning across it. (This can result in all tracks being collapsed, if they’re all empty.)
So there is the main difference between them,
auto-fit collapses empty tracks and redistributes the remaining space. As seen in the Rachel's example page.
auto-fit when you want it to fill remaining space.