Taken from Blizzard (and not only
.gif)
).
It goes as follows:
* Normal weapons are equally effective against all types of units.
* Concussion/Plasma weapons do 50% damage against Medium units and 25% against Large units.
* Explosive weapons do 50% damage against Small units and 75% against Medium units.
Note: Protoss shields take full damage from all kind of atacks, and buildings are always considered as "Large" (that's why units like the vulture takes a load of time to destroy it).
There is also another type of damage class, the splash damage. The atack of units such as the Sieged Tank provokes a small explosion around the atacked unit that will damage nearby units.
There's also the Lurker's and Firebat's type of splash damage, in a line.
--Edit--
Check here for all units sizes and attack type:
http://www.battle.net/scc/GS/damage.shtml