For me, Responsive Web Design is a relatively new term. Basically it refers to a web design layout that alters to it's environment. So the layout can alter depending on the screen size or resolution. This is not the same as a fluid design though, with Responsive Design the lay can actually change rather than just scale down.
I really like the idea of Responsive Web Design but it does seem a lot of effort, so I intent to start messing around with some CSS frameworks that embrace it.