Is the Witch Doctor's skill Locust Swarm ranged or melee? Can I hit someone across the screen if my pointer is on them, then the swarm goes to that spot or do I need to be right in front of the target and the target has to get hit with the animation? Also, is there a limited range for the "jump to additional"?
The maximum distance isn't very large, so it's basically a melee-range skill, but it actually functions like a short-range cone-shaped AoE spell.
There is (of course) a maximum range for the 'additional targets' effect, otherwise you could hit everything on the entire level with a single cast. That range is much larger than the initial casting range, however.
With those two factors in mind, by far the best way to use it is to cast it on a weaker (minion/summon) and let it jump to stronger targets.
Bear in mind that like all DoTs, re-applying it won't make it stronger, so you only need to refresh it once it's run out.
Locust Swarm is considered ranged, since it is a projectile, but you have to get pretty close to use it. You can't hit someone across the screen.
And to answer your second question, there is not a limited range for it to spread to additional targets. Note that the spread won't be unreasonably large, as monsters have to be close to each other for locust swarm to spread.