The ccTLD a specific type of top level domain (TLD) intended for specific countries. The two letter code used in country code Top Level Domain corresponds to the ISO3166 country codes definition standard.
ccTLD's are mostly intended for the use in and by the corresponding country and registration of these domains is sometimes restricted by the registar in the country. The leading search engines like Google and Bing use these codes as an indication of the target geographic region for the website. From an SEO perspective it is therefore important to use the ccTLD as it is intended to be used. For example, don't use the .co TLD as a substitute for .com as this TLD is intended for Columbia. For more information watch the video below:
Should I use ccTLDs for sites not targeted to those countries?