Evocation is the quick-and-dirty method of using power; it’s basically the art of slinging magical energy around more-or-less directly. If the wizard has a high level of control, evocation can be efficient and deadly. If he can’t control it, then it’s inefficient, but still very possibly deadly—to himself and everyone around him.
In evocation, you gather your power, then shove it into a temporary construct of will to control its direction and effect. This covers magic such as the simple point-and-blast (or, in the case of higher skill, point-and-laser). It is quicker but less certain than thaumaturgy, and it doesn’t need any physical constructs to assist it (though certain tools—like staves, rods, and wands—can help direct the energy). If you try to draw too much power or can’t focus sharply enough on your virtual construct, then the release of energy shatters the construct inside your mind (resulting in unconsciousness, if you’re lucky) or breaks loose on the physical plane, expending more power and creating a much more dramatic effect than planned. This is why you don’t want to use evocation to call fire inside an old house unless you’re good at it. The arrival of a fire brigade tends to be very disruptive to arcane investigations.
While evocation doesn’t actually require physical aids or focusing items, many evocationists do use focusing tools for an added bit of assistance. This may range from a chalk line quickly drawn to help visualize a boundary, to a trusted sword used as the directing point for a stroke of lightning, to a blasting rod spelled to make fire easier to control once conjured. Wizardly purists may hold that the true evocationist needs only his will, but actual practitioners in the field have a high regard for their skin and tend to err of the side of caution.
Some casters refer to it as "Quick and dirty" magic. It draws directly from the wizard's will and is turned loose without benefit of a guide or limit. It is dangerous and difficult below the Veil but is useful for throwing around a lot of energy.