The two or three letters succeeding the final ‘.’ in the URL of any web address is coined as the domain extension or top level domain (TLD), viz. ‘www.google.com’ the Google is called the domain name and ’com’ is called the top level domain name. Furthermore, there are sub types of TLDs, broadly speaking, generic TLD (gTLD) and country code TLD (ccTLD). Now, once the domain name has been chosen carefully, what comes next is getting a suitable TLD to enhance the search results and in accordance to the availability of the domain name.
gTLDs:
.com, .co, .net, .org, .ac are few TLDs to name, among many others, falling under this category. Each of these has a different purpose, search engine response and cost.
A website built for commercial purpose, a business venture and likes, mostly opt for ‘.com’. It is an abbreviation derived from the word commerce. Since it is the most commonly used extension, the search engine also lists a number of websites with this extension of domain name for a relevant search.
‘.co’, often confused as the country code top level domain for the country named Colombia, is actually the short for ‘company’ or ‘corporation’. However, in spite of this, most companies opt for .com because of its better search engine compatibility, as stated earlier. Proper SEO of web pages can do just fine while using ‘.co’ domain extension. Owing to this .co is also a costlier option compared to .com.
With everyone wanting to have a website for a commercial purpose with same domain names ‘.net’ became a nice option. This is the second best option for your business if you cannot get a website with ‘.com’ and you are not ready to alter your domain name. Some erudite in the field say that this has a direct impact over the customer since this extension cannot be worked with SEO as cited above. Howbeit, this has gained the popular choice if ‘.com’ is taken.
‘.org’ and ‘.ac’ stands specifically for non-profit organizations and academic institutions respectively.
ccTLDs:
As the Wikipedia defines, this is an Internet top-level domain generally used or reserved for a country, sovereign state, or dependent territory identified with a country code. The user needs to have a local presence in the area to be able to use a particular ccTLD. For example, .in is for India, .ca for Canada, .de for Germany and so and so forth.
Apart from this, there are several other TLDs reserved only for certain bodies. In order to use them, special permissions and documentation is required. The usage of .gov is only for government organizations, .mil is for military organizations are a few to state.