So, You want To Know What An Extension Is...
An extension is what comes after the period in the main part of a domain name.
Extensions come in a variety of flavors today such as:
There seems to be more coming along each day.
What is the difference?
The top three bolded are referred to as TLD's (Top Level Domains). The non-bolded extensions are referred to as LLD's (Lower Level Domains). The extensions use to stand for specific website purposes. Today most have lost any exclusivity execpt the last two in italics are reserved for educational institutions and the government even though they are considered TLD's.
What about international extensions?
Domain names must have a designation for the country of orgin. Every country has it's own unique identifer. This is why you see some addresses look like this: www.YourWebsite.co.uk The .uk extension is showing that the United Kingdom is where the domain is registered. Likewise www.MyDomain.co.ca would be a Canadian registrered domain.
In the United Stated we are privileged not to have to designate a country code extension because without a designation the default is that the domain is from the US even though the .us is a US country code extension.
Which one to choose?
No question a TLD (.com, .net, .org) is always preferable with a .com name being the real jewel and definetly more preferable than all the rest. The reasoning for this is that the default extension is always a .com.