Magicians are people, who are able to use "magic tools". Not every human can use them, so only those who can are called "magicians".
Many of the magic tools used by C3 are produced by Tinker. Key element of using them seems to be the ability to imaginate what the tool can/should do.
The keys can create a barrier - so no one else can enter it - except for them who are already inside. But you also can lock somebody inside this barrier - like Mahiru and Kuro were locked in the car.
The keys are the easiest form of magic tools. Yumikage, Tsurugi and Junichirou explain their usage like: "Imagine putting it into an invisible keyhole and release the dark powers hidden inside you with this key!" (Chapter 46)
Yumikage uses guns as weapons. Like Tinker explains, they have also created guns, that shoot beams of artificial sunlight - what makes all subclass (exept for Tsubaki's) die.