Alexa.com, Linux.org, WebRevenue.co, eLearningEuropa.info, Yahoo.co.uk, all examples above end with a different ‘extension’ – .com, .org, .net, .biz… and so on. We call this “extension” as top level domain (shortform:TLD).
Examples of other TLD include .uk, .ws, .co.jp, .com.sg, .tv, .edu, .co, .com.my, and .mobi. While most of these TLDs are open for public’s registration, there are strict regulations on certain domain registration. For example the registration of country code top level domains (like .co.uk for United Kingdom) are restricted for the citizens of the corresponding country; and the activities with such domains website are ruled by local regulations and cyber laws.
Certain extensions of these TLDs are used to describe the ‘characteristics’ of the website – like .biz for businesses, .edu for education (schools, universities, colleagues, etc), .org for public organization, and country code top level domain names are for locations.
And that’s not all. We now have more than 1,000+ generic TLDs (gTLD) opened to public, including .BAR, .FOREX, .CLUB, .COLLEGE, .REST, .WEBSITE, .WIEN, .XYZ, and so on.