RPC over HTTP is a way of connecting an Outlook client on a user’s desktop to a Exchaneg server over the HTTP (or WWW) protocol. It allow Remote Procedure Calls (RPC) to be transmitted on top of the HTTP protocol and helps to limit a server’s exposure to some of teh nastyness of having other ports open to the server.
Outlook Web Access is a completely different animal. It is basically a web page that allows you to access your e-mail remotely from another location (again, over HTTP) but it is displayed entirely in a web browser. Outlook web access has made soem great strides in the last two versions to make it much much more like the actual Outlook client– my personal favorite is access to the global address list. But it still lacks some of the features that are available in outlook itself.
In short, OWA is better suited for a quick use at a strange location where you have Internet access but don’t have your system available. HTTP over RPC would be ideally suited for a telecommuter or a roaming user that has their system with them at all times.