When Cassini (the astronomer) discovered Iapetus, he had a problem: he could only see it on one side of Saturn and not the other; years later, he saw it on the other side, but he needed a bigger telescope: it was only one tenth as bright.
Why? Well, obviously it was bright on one side and dim on the other... but we had to wait till Cassini (the orbiter) got to Saturn before we had any decent pictures that explained any further. (Arthur C. Clarke made this differential an important plot point of his novel 2001. Clarke may have been closer to the truth than he knew... but I'll get to that.)
So as it turns out, there's a huge dark splotch taking up most of the trailing side of Iapetus, like somebody hit it with a really big reddish-brown paintball. What's the splotch?
Well, as near as we can tell so far - based on Cassini flying close by the moon a couple times - somehow some dark material got deposited on one side, maybe from a collision. But you can only get so much deposit from a collision, so something else happened: from the time of the collision till now, it has been in a very gradual feedback loop. Dark stuff heats up better in sunlight, so the darker, relatively warmer side causes a very small amount of water to evaporate (still really cold - hundreds of degrees below zero - but ice is funny this way; tiny amounts will evaporate no matter how cold it is), and wind up deposited on the leading side, making the trailing side darker and darker, and the leading side as bright as water ice. Verrry gradually: this took billions of years.
It happens on Iapetus and not anywhere else because Iapetus is so far from Saturn, and thus rotates so slowly: it's seventy-nine Earth days from one sunrise to the next, which gives enough time for the little bit of evaporating water to migrate around the moon. (At Iapetus' surface temperature, the amount of water that evaporates is really small, but not zero. And more evaporates on the dark side than the light side.)
So there really is a dark side of *this* moon.
That's not the weirdest terrain feature on Iapetus, though. Around the equator (yes, the equator, almost precisely) there's a *huge* mountain range (as in 15-mile-high mountains): the ridge goes continuously through the dark side, and is an irregular line of peaks on the bright side. Why right on the equator? Why almost all the way around? Why irregular on the bright side but continuous on the dark side? You may have heard this refrain by now: We don't know the answer yet.
We find some fun things about these mountains, though. Iapetus has had landslides off those mountains, and the landslides behave differently than the ones on Earth; we think part of the reason is that the "rock" involved is largely water ice. The landslides scooted horizontally way farther than they would have on Earth, even accounting for the lower gravity: it's because they compressed the ice enough to melt it, and so it got slippery... like a cross between a landslide, an avalanche, and a glacier.
Iapetus isn't just odd for what it is. It's also a little odd for where it is. Remember how I said most moons of Saturn orbit around its equator? From that vantage point you can't see the rings. All the gas giants also have a bunch of what are called small "irregular" moons - rocks, probably asteroids or comets, that the gas giant captured; they're small (dozens instead of hundreds of miles in diameter), and orbit in highly elliptical orbits way far away from the planet.
Not Iapetus. Iapetus is virtually the same size as Rhea, so it's Saturn's third-largest moon, and it's the second-largest moon in the Solar System not to orbit above its primary's equator. (The largest such moon? Ours. The third largest? Phoebe, which is a quarter of Iapetus' size and also orbits Saturn, way further out even than Iapetus.) And Iapetus' orbit is almost precisely circular, not elliptical like "irregular" moons... at its distance, this circularity is surprising of itself.
So if you're thinking of finding a nice isolated place (with a good view) to retire to someday, Iapetus is the only world in the Solar System with a magnificent naked-eye view of Saturn's rings. From Iapetus, Saturn takes up about two degrees of the sky - four times Earth's full moon, or the same size as the Earth looks from the Moon: about the size of a half-dollar at arms' length. Since Iapetus has an inclined orbit, over its 1900-hour day you'd see Saturn's tilt change: you'd see Saturn go from full to new to full, her rings go from open to in-line to open the other way; you'd see the shadow of the rings on the planet; you'd see all the inner moons of Saturn as they orbited her.
And there's water there. Dig a little cavern to make your home, put out some big solar panels for heat and power, grow your food in your cave, and then go out in your space suit and ski some magnificent mountain peaks while enjoying the best view in the Solar System.