A gravitational lens is an effect of gravity upon distant light sources. Celestial objects that emit strong gravitational fields can act like a lens redirecting light rays. The reflections from gravitational bending of the path of light rays is predicted by Einstein's general theory of relativity.
The effect can be seen when a massive object (like a galaxy or black hole) is positioned between a light source and the observer. In such cases, the light source can frequently still be seen even if completely blocked from view. This happens because the light is bent by gravity making it appear as though the light source lies just to the side. But rather than creating a single image, the gravitational lens often creates multiple images. If the galaxy were perfectly symmetric with respect to the line between the light source and the Earth, then we would see a ring of light.
The bright light emitted from quasars is frequently observed demonstrating the process of gravitational lensing. A gravitational lens can be used to estimate the distance to the actual quasar using simple geometry if the mass and distance of the galaxy causing the lensing effect is known.
Classes of Lensing
There are three classes or types of gravitational lensing, which are:
- Strong lensing: Easily visible distortions of the light of distant objects which cause the formation of Einstein rings, arcs, and multiple images.
- Weak lensing: Many large objects must be grouped together and are observed to only distort by a few percent. This is seen as stretching of the background objects perpendicular to the direction to the center of the lens.
- Microlensing: Distortions are not seen but the amount of light received over time changes. 
- Gravitational Lensing by Pete Newbury for the University of British Columbia, Institute of Applied Mathematics