As light passes the border between media, depending on the relative refractive indices of the two media, the light will either be refracted to a lesser angle, or a greater one. The angles get measured with respect to the normal line, represented perpendicular to the boundary. In the case of light traveling from air into water, light would be refracted towards the normal line, because the light is slowed down in water; light traveling from water to air would refract the other way, instead.

Refraction between two surfaces is also referred to as reversible because if every condition's the same, the angles would be the same for light propagating in the opposite direction, too.

Snell's law is pretty much only true for isotropic or specular media like glass. In anisotropic media like crystals, birefringence can split the refracted ray into two rays, the ordinary one that follows Snell's law, and the other extraordinary one which might not.