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