In a number of countries, .co is used as a second-level domain to mean "commercial" and domain registrants register second-level domains of the form .co.xx, where xx is the country code top level domain (e.g., .co.uk in the U.K. and .co.jp in Japan).
Some of the countries using .co as a second-level domain include India (.in), Indonesia (.id), Israel (.il), the United Kingdom (.uk), South Africa (.za), Costa Rica (.cr), New Zealand (.nz), Japan (.jp), South Korea (.kr) and Cook Islands (.ck).
.co is also a top-level domain analogous to .com, .net and .org. Designated to the country of Colombia, .co was launched globally in 2010 as a generic country code top level domain gccTLD.
References
.co (second-level domain) Wikipedia(Text) CC BY-SA