Treated salt is simply rock salt (could be bagged or bulk) that has had something applied to it which usually lowers it's freeze point and improves its melting capability in some way (depending on what it is "treated" with). Some treatments result in the salt being less corrosive because what it is treated with is noncorrosive and by saturating and coating the salt, the result is a less corrosive product. Since the freeze point is lowered, there is less "freeze/thaw cycle" which is the prime culprit in concrete damage. So the result is less damage to concrete. It will tend to not "burn" vegetation and is less irritating to pets' feet. However, some treatments have the opposite effect and are actually more corrosive than ordinary rock salt.
Cost varies, but is naturally more expensive than plain rock salt.
Better? Depends on what you're looking for (see first paragraph).
