The CTLDs Distribution Map in the Overview tab of Site Explorer shows the distribution of referring domains that have links pointing to your site per country. 

A country code top-level domain (ccTLD) is an Internet top-level domain generally used or reserved for a country, a sovereign state, or a dependent territory.

Mouse over the map will give you CTLDs in number and distribution percentage:

It is to be noted also that for the CTLDs map we take into consideration only corresponding country top level domains. For example, for the United States it will be .us domains (.com, .net, .org domains do not refer to CTLDs).