Domain names are read from right to left: the root, then the TLD, then the name registered under it. IANA maintains the list of TLDs in the root zone. The main kinds are generic TLDs (gTLDs such as .com, .org and the hundreds of new gTLDs delegated since 2013), country-code TLDs (two-letter ccTLDs such as .de or .tr), sponsored and restricted ones such as .edu and .gov, and the infrastructure domain .arpa.
Some registries sell names only or mainly at the third level, under zones such as .co.uk, .com.tr or .com.au. Those zones are not TLDs, but they behave like them for registration purposes. The Public Suffix List, a community-maintained file, records them so that browsers and tools know that example.co.uk, not co.uk, is the registrable domain.
The TLD determines which registry's rules apply, which registrars can sell the name, how disputes are handled and whether registration data is available through RDAP, WHOIS or neither. OrbitProbe's TLD directory lists these facts for the best-known extensions.
Example
www.example.com.
└── com = top-level domain
└────── example.com = registered (second-level) domain
└── www = subdomain