The most basic function of a camera rail system is to add physical attachability to your camera. The most basic systems invlove two rods which attach to the camera via a camera plate which can either be a quick release plate which makes it easily detachable, or a non quick release which is fine if you plan on keeping the rails more or less always attached. And as can be expected rail systems have standard 1/4 20 threads on the bottom to attach to a tripod, jib, or other support system.
One thing to look out for when buying a system is whether the camera block and the 1/4 20 mounting block are separate or attached. Most systems are a single block, but some have separate blocks so that you can adjust the balance of the rig.
Next are the rods themselves, which come in two different thicknesses, 15mm and 19mm. For most rigs, 15mm are more than enough and more common. But for larger rigs with more weight, 19mm can be used, just keep in mind both the camera block and all the accessories need to be able to accomodate the right thickness. They can also come in different materials: Steel is great all around but can add a lot of weight, aluminum is great for its reduction in weight, and carbon fiber is both stronger than aluminum and lighter than steel. For added strength you can get solid core rods, but most do just fine with hollow cores. I've never actually heard of a rod breaking, so the only real concern is bending, you want a nice stiff rig. So especially if you're looking at 18" or 24" rods, you'll want to make sure you get rods that are strong enough to resist bending.
So what do you put on rails? Most common are the Matte Box and Follow focus. You can also put on hand grips and a shoulder pad, and to help out the balance throw a counterweight in the rear. The idea when going shoulder mount is to have the center of gravity be right on your shoulder. You can also add a cage that wraps around the camera giving you more mounting points, maybe some extra rods that attach to give you more ideal positioning for your hand grips or other accessories.
Another thing to keep in mind is that many accessories are built for a standard lens height, that is, the center of the lens should be a specific distance from the rails. There are plenty of rigs and accessories which have height adjustability, but some don't, so keep that in mind.