Phototropism is the movement of plant structures in response to light. Phototropism may be positive or  negative. Positive phototropism is that in which the plant movement (or growth) is towards the light source and negative phototropism is that in which the movement (or growth) is inverse, away from the light source.
Phototropism relates to auxins since the exposition of one side of the plant to light makes these hormones concentrate in the darker side. This fact makes the auxin action upon the stem to be positive, i.e., the growth of the darker side is more intense and the plant arcs towards the lighter side. In the root (when submitted to light, in general experimentally) the auxin action is negative (over the positive range), the growth of the darker side is inhibited and the root curves towards this side.