The big moon is Titan, and by big, I mean bigger than the planet Mercury. Big enough to have a thick nitrogen atmosphere, clearly visible in this picture. The bright moon superposed right on top of Titan is Dione, its icy surface shiny and white.
On the right, just outside the rings, is tiny, flying saucer-shaped Pandora. And the fourth moon? That’s Pan, the tiny white spot in the gap in the rings on the left, barely visible in this shot. But that’s understandable, since Pan is less than 30 km (18 miles) across, and this was taken from a distance of nearly 2 million kilometers (1.2 million miles) away! (Source)