Stephan's Quintet is a group of five galaxies. The four physically associated galaxies are called Hickson Compact Group 92. Those four galaxies are NGC 7317-7320. They lie at an estimated distance of 290 light-years from us. The fifth galaxy is much closer to us, and lies at a distance of roughly 40 million light years from us. It is a spiral galaxy depicted at the bottom of the image.
The centers of the galaxies appear as bright yellow-pink knots inside the blue regions of stars.
The magnificent green arc seen in the pictures is believed to be a giant shockwave that travels through intergalactic gas. The shockwave is caused by an emission of hot hydrogen gas from galaxy NGC 7318B into the center of the group. This wave is moving at a speed of several millions of miles per hour. Some of the gas between the galaxies heats up to temperatures of millions of degrees as a result of this emission. This shockwave is larger than our Solar System in size.
Sources:
NASA
http://en.wikipedia.org/wiki/Stephan%27s_Quintet