Generally speaking, your vision can be divided into your central vision and your peripheral vision. Your central vision is relatively easily tested (for example, with a standard Snellen (eye) chart. Your peripheral vision is a little more difficult to test.
Visual field testing usually involves testing the peripheral vision. There are several ways this can be done. Northern Eye Surgeons uses the gold-standard Humphrey Visual Field Analyser. This test involves you sitting in front of an illuminated dome. Lights of varying intensity are then projected into the dome, and you press a button whenever you see the light. This test is difficult and requires a lot of concentration, as the desired effect is to try and find out how good (or bad) your peripheral vision is.