Microsoft's remote desktop protocol (RDP), virtual network computing (VNC), and other remote desktop access software allow users to remotely control another computer. Popular uses of the technology include remote technical support, and accessing files on one's work computer from one's home computer, etc. The software transmits the keyboard presses and mouse clicks from one computer to another relaying the screen updates back in the other direction, over a network.
Due to the way these applications work and security concerns related with them the CS department blocks the protocols at the firewall.