A shield sacrifices itself and completely blocks any single attack from the following sources, regardless of the damage amount: vek attack (including Psion Tyrant damage), friendly weapons, emerging vek, collision.
Shield blocks following effects without sacrifice itself: fire, acid, freeze. (Applying shield on units already affected by those effects doesn't remove them, and won't prevent fire damage)
Cast shield onto frozen unit will make it both frozen and shielded which is resistant to two attacks.
Shield cannot block any of the "kill" effect including self-destruction, satellite launch, fall, train collision, air-bombing and other environmental kill effects.
The shield persists through turns indefinitely, only disappearing once an effect removes it.
Shield can be applied on Mechs, Veks, structures, other allied/enemy units and moutains.
Shield can be applied on train but it's considered one shield even though train has two units. Attack on any part of the train removes shield from whole train and conversely, applying shield on any part of the train grant shield on the whole train.
Intuitively, when combo damage (e.g. damage + push collision) happens in one move, first damage breaks shield and the second damage will land on the unit. It's important to note that AOE damage like lightning whip, hits a train twice thus will damage a shielded train.
Abilities That Grant Shields
"Autoshield" module that makes a building get a shield after it has been attacked once.
Shield Projector Weapon, for Science Mech Class, e.g. Defence Mech in the Zenith Guard Mech Squad.