Data service generally implies some type of AMF serialization of the message provided by the backend. An httpservice call is just that, an httpservice call.
I generally see httpservice calls used for loading xml and text documents.
Data service is probably a server pushed communication like messaging.. It carries lots of changes as an object or arraycollection and as ubuntu said something like asyn service calls..
and http service is like traditional http service .. we can use http services to load remote resources also in flex application..