Category importing is similar to product importing. Here is an example of a working import file with the minimum required fields - note that the 'url_path' field is used to determine how categories are arranged in the catalog.
is_active,name,url_path Yes,"Hats",hats.html Yes,"Top Hats",hats/top-hats.html Yes,"Wide Hats",hats/wide-hats.html Yes,"Silk Top Hats",hats/top-hats/silk.html Yes,"Goggles",goggles.html
This produces this layout:
Default Category |-> Hats | |-> Wide Hats | `-> Top Hats | `-> Silk Top Hats `-> Goggles