I think I understand it - basically the idea is that it provides the features of Bitcoin but is specifically set up to make it easy to exchange fiat currencies. So it's kind of like if you want to trust a guy with USD he'll pay out those dollars to you for your GBP, then you can exchange that fiat instantly just like you can transfer Bitcoins instantly.

For me this limits the 'risk' people talk about when they say it 'depends on trust'. All currency exchanges depend on trust, including when you send someone bitcoin for a product - if they don't deliver it, they broke your trust and you're out of that money. Difference is, money isn't supposed to stay in ripple for long, you're probably just making an exchange for fiat.