Please note, we don't use terms interchangeably. Armor and heavy defense are completely unrelated to one another, same as shields and forcefields are not the same thing -- we're really precise with our language when at all possible, to avoid such confusion (my long background in business software encouraged this heavily).
That said, there is a specific class of ship called the Armor Ship. It's like bulletproof fighters, or anti-armor, or whatever -- that's it's name. Anti-armor ships are also good against a variety of other ships, though I couldn't list them all off the top of my head. It's a good unit, and some folks' favorite, but it depends on the opposing ship mix as to how powerful any specific ship class is really going to be in your game.
Regarding force fields, those are their own separate thing (though almost everything that is good against heavy defense is also good against force fields, and vice-versa). Each turret class is its own thing, and is unrelated to heavy defense. The "heavy defense" is stuff like fortresses, raid engines, and almost all of those big, specialized fixed-position ships that the AI can get. Ion cannons are another example. Black hole machines, alarm posts, radar jammers... all that sort of thing is heavy defense.