In Starcraft II there are certain spots on the map where Zerg players like to hide Overlords. I imagine that they are on higher level just as the main base is usually on a higher level than the natural expension on most maps. How does the mechanic work? It is clear that any ground unit can't see anything above its level (units that are on the high ground of a ramp), but does that still apply to air units or are those spots special and have therefore special rules?
In Starcraft 2, ground units can only see things on elevations equal or lesser to the elevation they are themselves. Air units can see things regardless of elevation.
An air unit, like an overlord, above one of those pillars is on an elevation greater than the elevation of any nearby ground units. Since there is usually no nearby location that is equal or higher elevation to the pillars, the only way to see the air unit is to bring another air unit nearby.
These pillars are just small areas of higher elevation. They're also impassible to ground units, so you can't plop down a siege tank up there. They have no other special rules.
Think of a vertical column above those spots which is why you can't see the overlords. (Doesn't apply in real life, you can diagonally see the overlord in those spots, but in the context of Starcraft you cannot).
All units have a field of view on the plane they are on. If you have something obstructing your view on the low ground, that obstruction goes straight up as well. However if you have a flying unit of your own (or building as Terran), your flying unit provides the view above that obstruction thereby revealing it to your low ground units which makes it vulnerable to attack.