The top-level (io) stands for the British Indian Ocean Territory that has a population of 4,000. They've opened up the .io top-level domain for international registrations, like the Cocos Islands (.cc) and many others have done. It's now big business for small countries with cool abbreviations to do this. And it's lucrative ... consider the €60 I just paid for one year's registration, compared to about €5 a year for a .net.au or .com, etc. Thinking relatively though, it just means that I have one less cafe latte every week -- which is a good thing.