The Firefly 'verse equivalent of a tractor beam -- a magnetic grappler is much more... tactile. A large clamp (looking like nothing so much as the huge magnet that salvage yards use to pick up automobiles, but with larger outer flanges) is deployed and fired like a grappling hook at another spaceship. This grapple is on a tether, which is then reeled in. A few facts, or inferred facts:

• The grapple has a relatively short range. (In the chase scene in Serenity, we don't see the Reavers ready the grapple until they are within... let's ballpark it and say 10 ship lengths -- roughly 2000 feet or 600 meters.)
  • This shorter range might be atmo-only (or when in a gravity field). Range would really have to be quite a bit further in open space, simply because distances are greater.
• It seems to have forward firing arc. (At least as seen on the Reaver ship in Serenity.
• It is very unlikely that breaking away is possible once the grapple has fasted onto the ship -- certainly we are led to believe that a direct hit from the grapple would be the end of the line for Serenity and her crew.
• Reavers are notorious for the use of the magnetic grappler -- this is inferred: we don't see any other large craft using the device or threaten its use, and Wash's comment in the pilot episode ("there's their magnetic grappler") gives the impression that such a device is almost expected on a Reaver ship. (Certainly, pirate-style activities in the middle of space would require some sort of means of forcing the other ship to stop.)