An Einstein ring is a complete circle image of a background galaxy, which is formed when the background galaxy, a massive, foreground galaxy, and the observer are all aligned perfectly. One can be seen in this image of the gravitational lens 5921+0638 taken by the Hubble Space Telescope.
Gravitational lenses occur when light travelling towards us from a distant galaxy is magnified and distorted as it encounters a massive object between the galaxy and the observer. These gravitational lenses can effectively extend the range of a telescope, often allowing astronomers to peer much further back into the early Universe than they normally could.
Image Credit: NASA