From Market Ruler Help
The "suffix" of a domain separated with a period (or dot):
- .com is the TLD of http://www.marketruler.com
- .co.uk is the TLD of http://www.google.co.uk
- .to is the TLD of http://pw.ex.to/
Some TLDs include two "words" (.co.uk) and some include one. The reasons are largely political.