Burst:
If a target have a total cover from the point of origin of the spell (and no necesserly from the caster), than it is not affected.
Spread:
Spread are like burst, but with one difference, spread can turn the corner, so the total cover donèt apply. If for a 20' spread, there is a way to travel from the origin to the target, travelling no more than 20', than the target is include.
Exemple:
O
---------\
T
If O is the origin and T a potential target. In the case of a burst, T is not affected, in the case of a spread, if the ditance to travel from O to T is equal or inferior to the spread radius, than T is a target. If it is greater, than T is not affected.
Hope it is clear.