Imagine your user is accessing a content hosted in AWS Europe from Singapore, it takes ~120 milliseconds as latency for this delivery, whereas if the same content is delivered from AWS Singapore using Amazon CloudFront, the latency can be reduced to ~10 milliseconds. If your site is content heavy multiply the above with number of requests, hops etc and imagine the latency savings that Amazon CloudFront can provide.
Internally Amazon CloudFront maintains persistent connections with the content origins so that changes/addition can be picked quickly.
Following diagram illustrates current list of edge locations offered by Amazon Web Services. Keep watching http://aws.amazon.com/cloudfront/#details for updates, more edge locations around the world are on your way.