"Campaign" means the unit is not a standard unit and is only found in the campaign and custom maps, while "Melee" means it is a standard unit. "Campaign" units often vary from the description of the group (Undead Scourge, for example, has a Pit Lord listed under it). "Special" is usually a summonable or upgraded unit (Spirit Wolf or Troll Berserker).
Note that in Warcraft III the playable races actually referred to factions, and that many other races are included within this playable race.
Flying Unit Improvements: We have turned off collision detection for flying units so that they can now easily pass through each other. This will allow air units to move about more smoothly than before. We have also added "separation behavior" so air units will break out of clusters once they reach a destination.
When "attack-moving", air units now prefer to attack other air units more often than previously.
Units in a transport over ground will "spill out" over a wider area. This means that surrounding a webbed transport and killing it will generally not kill the passengers.