Domain hacks
A hack uses the extension as the end of the word: shortly becomes short.ly, delicious becomes delicio.us.
One name, many extensions
Hacks are suggestions: most are not checked until you press Check. “Available” is confirmed with the registry where the registry publishes a lookup service; for the rest, “probably available” means the name has no DNS records, which is evidence, not proof. Some extensions (.us, .it, .eu, .ca) need you to live or be based in the country or region. Prices are public list prices and change; always confirm with your registrar.