Aberrations are optical imperfections that impair the resolution performance of a lens. The shorter the wavelength of light is, the more it will be refracted by an optical surface. As a result blue light has a shorter focal length then red light. The picture shows a lens focusing a white light point source. At the point where green light is focused, the red and blue light is blurred.
Spherical aberration occurs when the edges of a lens refract more light than the center.
The effect of spherical aberration is that if you view an image of a point source, it will have a diffuse halo around it. More expensive lens systems compensate for more colors. This causes an image magnification error - either a pin cushion or barrel distortion. Like the other kinds of aberration, this can be minimized when the design includes compensating lenses. There is a strong relationship between the amount of aberration a lens will display, relative to its Numerical Aperture.