The government of Tonga sells domains in its ccTLD to any interested party. The .to ccTLD is administered by the Tonga Network Information Center (Tonic).
Because to is a common English preposition, it became popular to craft memorable URLs called domain hacks that take advantage of this; URL shortening/redirection services (e.g. go.to) are a popular use. Other domain hacks do not use the to as a preposition, including Daniel J. Bernstein's website cr.yp.to, London web development company Potato p.ota.to, and uploading services like uloz.to (which means "save it" in Czech and Slovak).
"T.O." is also a common nickname for Toronto, Ontario, Canada and is used as a city domain extension as exemplified by popular Toronto sites www.dine.to and www.jobs.to . This domain extension is also used for the city of Torino (Turin), Italy.
As the .to domains are purchased, all normal DNS operations are possible and registered sites are not required to display ads on behalf of the registrar. Some domains are free, like .edu.to, but only to real Tongan educational institutions. At this moment businesses registered in Tonga can also get free domains. People who sell on .to domains can claim a commission.
.to is one of the few ccTLDs that (officially) do not maintain a (public) WHOIS database providing registrant information. It is possible to contact domain registrants via tonic.to by typing the domain in the domain search field under "New Domain Name".
Contrary to popular belief, the .tv ccTLD is not used as a homonym. This often leads people to believe that movie2k.tv is the same as movie2k.to, but they're not. .tv is actually the Internet country code top-level domain (ccTLD) for the small Pacific island nation of Tuvalu.