Instructive, because coming from a strong RPC perspective, Tim illuminates the distributed application problem with slightly different shades. I like this graphical model:
Every communication protocol has a state machine. For some protocols they are very simple, for others they are more complex. When you implement a protocol via RPC, you build methods that modify the state of the communication. That state is maintained as a black box at the endpoint. Because the protocol state is hidden, it is easy to get things wrong. For instance, you might call Process before calling Init....The essence of REST is to make the states of the protocol explicit and addressible by URIs. The current state of the protocol state machine is represented by the URI you just operated on and the state representation you retrieved. You change state by operating on the URI of the state you're moving to, making that your new state. A state's representation includes the links (arcs in the graph) to the other states that you can move to from the current state.