A top-level domain or TLD belongs to a high level within the categorised Domain Name System on the world wide web.Such a domain name is generally installed within the root zone of any given name space. On the other hand, low levelor second level domains constitute of thelast part of the domain name within a qualified domain name system. 

In a given domain name www.example.com, .com is the top-level domain.
 
Certified organizations under the Internet Corporation for Assigned Names and Numbers (ICANN), manage the responsibility of management of almost all top-level domains. These are delegated for the operation of Internet Assigned Numbers Authority (IANA). Such organisations take up the charge for maintaining certain DNS root zones.

As you register a domain name for your website, you can select from a plethora of domain options. Some popular top level domains are as follows:

- .com
- .net
- .org 

They are extremelypopular but tough to find. 

Some second level domains are as follows:

- .asn.au
- .com.au
- .net.au etc.

They are not as popular as TLDs because one cannot find choicest second level domains provided top level names have been already taken. 

How are second level domains different from the top ones?

Most Internet users treat .com as a genuine domain name. Your competitors might rely on name recognition and TLD usage heavily for advertising. As a result, visitors forget about the presence of your website because it uses a second level domain.

Sometimes desired domain names are not available with .com extensions. In such cases one has to go with .net or .org and second level extensions. Besides it is essential to weigh all options carefully while buying a given domain along with available extension. Sometimes .com domains are trademarked so that the company does not get into any legal problems by registering a domain name with different extension. Apart from the aforementioned trademark issues, one has to ensure that the domain name does not get hijacked after registration along with second level extension. 

So the fact remains that both top level domains and second level domains appear except for the www prefix. A subdomain appears like 'sub.example.com' and a domain looks like 'example.com'. In a multisite, mapped domain is used, here the subdomain points to mapped.example.com’. It is advisable to stay away from domain mapping plugins as far as subdomains are concerned. 

To make your website look attractive and also build its repute, you can use both the domains because you can see an attractive domain like example.co.uk. Example is not a subdomain here but the main domain whereas co.uk is the TLD. Check for domain name registration services with your web host for appropriate domain names.

Therefore, domain names are associated with internet users of a company as well as outsiders. Whether you use a TLD or a second level domain, ensure that it is easy to remember. Remember that with a top level extension, you can always obtain an attractive domain name. You can check with your domain registrar for the availability of choice able domains. Only then you can find the right domain name for your business.