I read this morning about a new feature on dnsimple.com called an ALIAS record. I found the language confusing, so I wrote up what I thought it was saying and ran it by Anthony Eden for confirmation. He says this is accurate.
DNS has a limit. The top level of a domain must be an A record, it cannot be a CNAME. For example, scripting.com cannot be a CNAME, it must be an IP address, because that's what the value of A records are. IP addresses.
However if I use dnsimple.com, I could make the value of scripting.com an ALIAS record whose value is s3.amazonaws.com. Then, when a request comes in to lookup scripting.com, it looks up s3.amazonaws.com, gets an IP address and returns it. The caller doesn't know or care how they got the address.