What's the difference between a TLD and a second-level domain (SLD)?

The difference between a top-level domain (TLD) and a second-level domain (SLD) lies in their positions within the Domain Name System (DNS) hierarchy and their functions.

### Top-Level Domain (TLD)
- **Position in the DNS Hierarchy**: A TLD is the highest level in the DNS hierarchy. It appears at the end of a domain name, immediately following the last dot (e.g., `.com`, `.org`, `.net`, `.uk`).
- **Function**: TLDs help categorize and organize domain names by type, purpose, or geographic location. They are managed by registry operators under the oversight of ICANN (for gTLDs) or national authorities (for ccTLDs).
- **Examples**: Common TLDs include `.com` (commercial), `.org` (organization), `.net` (network), `.edu` (educational institutions), and country-specific TLDs like `.uk` (United Kingdom) and `.jp` (Japan).

### Second-Level Domain (SLD)
- **Position in the DNS Hierarchy**: A second-level domain is positioned directly to the left of the TLD. It is the part of the domain name that typically identifies the specific organization, entity, or individual associated with the domain.
- **Function**: SLDs are used to create a unique domain name within the scope of a given TLD. They are chosen by the domain registrant and registered through a domain registrar. The combination of an SLD and a TLD forms a complete domain name (e.g., `example.com`, where `example` is the SLD and `.com` is the TLD).
- **Examples**: In the domain name `example.com`, `example` is the SLD. Other examples include `wikipedia.org` (`wikipedia` is the SLD and `.org` is the TLD) and `google.co.uk` (`google` is the SLD and `.co.uk` is the combined TLD for commercial entities in the United Kingdom).

### Key Differences
- **Hierarchy Level**: The TLD is at the highest level, while the SLD is immediately below it.
- **Role**: TLDs categorize domains broadly, while SLDs provide specific identification within a TLD.
- **Control**: TLDs are controlled by registry operators and are subject to ICANN regulations for gTLDs and national policies for ccTLDs. SLDs are controlled by the domain registrant who has registered the domain through a domain registrar.

In summary, the TLD defines the overarching category or geographic area of a domain name, while the SLD specifies the unique identity within that category or region. Together, they form the complete domain name used to identify websites and other online resources.