Einstein's theory of relativity states that gravity is a curvature in space and time. Objects travelling through space follow a straight line through space-time, even though it may appear curved. One implication of this theory is that all objects are affected by gravity, not just those that possess mass. This is a radical departure from Newton's ideas concerning gravity and motion. Einstein predicted that light, which is massless, will be bent by gravity. This prediction was confirmed in 1922 by measuring the position of stars near the Sun during a solar eclipse and comparing them with photographs taken at a different time of year.
The bending of light by gravity is very similar to the bending that occurs when light passes through a lens. The vast distances associated with stars causes them to appear as points. If two stars, in relation to the viewer, are in perfect alignment (that is, one is behind the other), will the more distant star disappear? Einstein concluded that both stars would be visible, but in a very unusual way. The gravity from the closer star would bend some of the light from the more distant star toward the viewer. Since the path of the light was affected, the more distant star would appear to the viewer as a halo with the closer star in the middle. Different degrees of alignment between the two stars would result in different lensing effects. This arrangement of bodies, where a "bender" amplifies and/or reshapes the image of a more distant object, is called a gravitational lens.
In practice, the probability of discovering a pair of stars in perfect alignment is infinitesimally small. However, a lensing effect can also be achieved using galaxies as benders. Astronomers are interested in gravitational lenses for several reasons. A bender the size of a galaxy can gather and focus far more light and radiation than a line-of-sight receiver on Earth. An object that has been lensed is often many times brighter than normal. This could allow us to view distant objects with greater detail and clarity.
The lensing effect is achieved by gravity and is indeterminate of the brightness of the bender. A lens can be used to estimate the curvature of space, and thus the mass of the bender, even if it isn't visible. Gravitational lenses can be used to estimate the number of galaxies in the universe. A large quantity of galaxies will statistically yield a large number of lenses. A thorough search for gravitational lenses may allow us to estimate the number of galaxies that are invisible to us.
Also, lenses can allow us to calculate the distances to the bender and the object. When the bender and object aren't perfectly aligned, a double image will often form. Since the radiation from the two images followed different paths through space-time, they will most likely have different lengths. The speed of light is constant so the images will appear at different times. One image will be an "instant replay" of the other. It is possible to take the angle between the two images and the time difference (with a little math) and compute the distances.
Gravitational Lens Monitoring Project - self exlanatory
Gravitational Lenses - examples of lenses
* Photo credit - W.N. Colley and E. Turner (Princeton University), J.A. Tyson (Bell Labs, Lucent Technologies), STScI, and NASA