What is Top Level Domain (TLD) & Second Level Domain (SLD)?
TLD's in general are often referred to as the Namespace. For example the domain name domain.info is part of the .info namespace.
There are three types of TLD's.
1. The most common are gTLD's (generic Top Level Domains), such as .Com, .Net, .Org., and now .Info. Anyone is authorized to register domain names in a TLD namespace.
2. rTLD's (restricted Top Level Domains) such as .Biz and .US only allow people who meet certain criteria to register domain names, such as Business related sites for .Biz.
3. ccTLD's (country code Top Level Domains) such as .in (India) are individually assigned to countries and their dependencies. Each country sets specific registration criteria.
Every TLD Registry - generic, restricted, or country code - has its own prices, policies, and procedures that Registrants are subject to.
Second Level Domain
In the Domain Name System the level of the hierarchy underneath the Top Level Domains is called "second level domains." In the example domain name "Domain.com" the "Domain" part is the second level domain. Second level domain names are what you register in the Top Level Domain Registry database using the services of the Registrar.
Domain Extension Meanings
Below you'll find some of the more popular domain extensions and the domain extension meaning.
.COM (Commercial) - Generally intended for commercial use. By far the most recognized domain name extension.
.NET (Network) - This was originally intended for network oriented sites such as internet service providers. This definitely isn't the case any longer. People will register .NET domains for commercial use, generally if the .COM extension isn't available or if they'd like to protect their main .COM domain by registering all available extensions.
.ORG (Organization) - This domain extension was originally intended for non-profit or trade organizations. Again, this is no longer the case and is generally registered by anyone looking to protect their main domain by registering all available domain extensions or simply because a better extension isn't available for registration.
.BIZ (Business) - This is a gTLD that is generally registered to classify their website as a business. Although it's not a commonly recognized extension, I feel that it's a great alternative if the .COM extension isn't available and for only $8.95 per year through www.WebsiteSpot.com it certainly wouldn't hurt to register the extension so that your competition doesn't try to squat on it.
.INFO (Informational) - This gTLD is generally used as an extension for informational websites. I believe that other than .COM, .NET & .ORG, it's the most popular domain extension used. If you are developing an informational website it's definitely a domain extension that I'd register.