Special modifiers are bonuses found on items in Diablo II like Uniques, Rune Words, and Set Items, but not on Magical or Rare Items. They are not like prefixes or suffixes. These bonuses are picked from a pre-set list, not randomly-generated, and not randomly valued either. There aren't multiple types or levels of Crushing Blow, or Ignores Target's Defense; it's there or it's not, to whatever value is preset. However, some special modifiers are granted by multiple items and can combine, for instance for a higher chance at a Crushing Blow.
These modifiers do not have Character level requirements or Item levels, like prefixes and suffixes do, and the character level required to use these items will always be pre-determined on Unique and Set Items. On Crafted Items the Character level can be pre-set, but is usually determined by the random mods (since they're usually higher than the minimum preset value).
In Diablo III some Legendary and Set items have similar special attributes.