A domain name is a name that is easily understandable by humans - like uk2.net . This is the URL that you would type into your web browser. All domain names are managed by The Internet Corporation for Assigned Names and Numbers (ICANN).
A TLD - or Top Level Domain - is the end part of the domain name. For uk2.net , the top-level domain is the .net . There are a number of common TLDs which include .com, .net, .org. and .info. TLDs can also point to geographical locations, these are known as country codes, for example, .uk represents the United Kindgom.
The second-level domain is the part of a domain name that comes right before the TLD - the uk2 part in uk2.net , for example.