What are Top Level Domains or TLD’s?
“A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet.”
— Wikipedia – http://en.wikipedia.org/wiki/Top-level_domain
The most common and popular TLD is .com. In the example of www.alpineweb.com, .com is the Top Level Domain.
It follows then in the example of www.alpineweb.com that alpineweb.com is a second level domain and www.alpineweb.com is a third level domain.