What is the difference between KRC / VNC type remote desktop and RDP
KRC/VNC and other remote control tools work by sending a picture of the desktop across the network. There are some optimizations, such as only sending parts of the screen which have changed, but that's essentially it - you see what is on the remote screen.
RDP, on the other hand, works by sending instructions on how to draw the screen to the client computer. Rather than sending a picture of a Window, the host machine sends a description of the window to the client machine, then the client machine is responsible for rendering an image and displaying it.
Since the client computer "understands" the image it has created for you, it can perform simple operations like moving windows without having to (a) send the mouse input to the host computer, (b) wait for the host computer to render the moved window, and (c) wait for the response. It can just calculate and draw the results for you right away.
This also allows the RDP session to be in different resolutions from the remote machine, as you are not looking at the remote desktop at all, you have a "session" which is not related to what is happening on the remote machine's display.