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 which do not use the to as a preposition include 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).
As the .to domains are paid for, 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 bonus.
.to is one of the few ccTLDs that (officially) do not maintain a (public) WHOIS database providing registrant information.