Shape Shifting is the ability that a Caster has. To be able to Shape Shift is to take the form of any tangible object. Not all casters are able to use this, because it's only seen with a minuscule amount of Casters. Barclay Kent was able to Shape Shift.
Evos and Shifters are the only casters that can genuinely shape shift. Evos are able to shift into anyone they want and bend the will of mortals, but not shape shift into an object. A Shifter can shift objects and change their appearance but it's not clear if than can actually change into the intended person. An Illusionist can only change the appearance of something, someone, or themselves, but not the actual object/person itself. Sirens can change their appearance, but not themselves, someone else's appearance, or into an object. When Evos morph it starts off as a tingle that starts in their toes that travels up tjeor legs and torso until it feels more like a sting, one that radiates across their entire body. When Shifters shift objects, the air shifts like heat waves rising from hot asphalt and the objects blur, transforming them into other objects.