q3_ui & ui - everything about menus
q3_ui is the base menu, ui is the missionpack backend with the menu's compiled at runtime within the vm.
unix - UNIX/Linux/Mac specific code for client and server
win32 - Windoze specific code for client and server
These two directories have been combined and are now sys/ with the icons moved to ../misc/, the directx backend for win32 has been replaced with SDL.