One of the more visually spectacular features of the universe, a ring galaxy is the result of two galaxies
colliding head-on at high speeds.
Ordinarily, galaxies collide at small enough relative velocities that they merge into one. However, if a fast-moving small galaxy travels directly to the center of a larger one (usually a spiral), it can simply pass through.
Galaxies are tenuous enough that direct collisions between their component stars are uncommon. There are significant gravitational effects between them, though. In the type of event producing a ring galaxy, the impulse of the collision draws the larger galaxy's stars inward toward its center. After the smaller of the two has passed through, the big galaxy is far from equilibrium, and the quickest way for it to return to a balanced state is for a mighty wave to propagate radially outward, taking most of the galaxy's gas clouds with it.
The high pressure of the wave triggers the collapse of these gas clouds along the expanding perimeter, and great quantities of stars form, many of which are very massive, and hence brighter and bluer than those in the main of the galaxy. Visually, a ring galaxy appears as a yellow disc at the center of a thin, bright, blue ring. The effect is quite spectacular.
The best-known ring galaxy is the Cartwheel Galaxy. There's an excellent image and description of it at http://www.seds.org/hst/Cartwheel.html.