From Halopedia, the Halo wiki
The Circle Menu is a control system featured in Halo Wars. It is used almost for everything such as constructing units, vehicles and buildings. The player can navigate through the Circle Menu via the Analog stick, to select whatever they so choose.
It was designed by the Ensemble Studios in order to solve the problem of cumbersome RTS controls on the console. On PC, the player has the ability to use mouse and keyboard at the same time, which makes multitasking easy (it also allows for a great deal of keyboard combinations, which improves the ability to control the interface and the field of battle). The Ensemble studios solved the problem by creating a fast and easy to control mechanism, the Circle Menu. The menu can contain up to 8 "articles", with units being on the right side and upgrades for them on the other. Additional information (such as cost, name) is displayed in the middle of the circle, with general information on the unit or upgrade currently chosen on the far right beyond the menu itself.
The Circle Menu is very innovative and truly enriched the RTS genre on the consoles and proved that RTS games can be easily and fluently controlled, even if we only use the controller.