What causes a potted plant placed near a window to bend outwardly?

Phototropism is a plant's natural response to light as a plant exposed to light from one side will grow towards the light source.  

Auxin a plant hormone is produced in the tips of the plant and helps to regulate growth. When light hits one side of a plant, it causes the auxin to migrate to the shaded side. This results in more auxin on the shaded side, which causes the plant to grow more on that side, resulting in the bending of the plant towards the light source.