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

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

You can click on the Country-code tab to see country code top-level domains (CTLD). These are Internet top-level domains generally used or reserved for a country, a sovereign state, or a dependent territory.

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).