A TLD stands for a Top Level Domain.
Back when the internet first started everyone only knew of the .COM, .ORG, .NET TLD extensions.
Now we have gTLD as well which stands for Generic Top Level Domains.
You will also see country based Top Level Domains such as:
.IN for India
.MX for Mexico
.CN for China just to name a very few.
Currently there are a total of over 360+ total TLD to date as of 7/10/2015
For more information on all TLD, and the registrars please visit ICANN https://www.icann.org/
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