I was all right as far as here (but what's an agent?)
Your basic sentence has three things: An action, something that DOES the action, and something the action happens to.
The something that is DOING the action is the agent.
If the agent is the subject of the sentence, then it's active:
Euclid threw the ball.
Euclid is the agent, doing the throwing, and the ball is the object, being thrown. "threw" is the action, or verb. Since the agent is the subject, this is active voice.
If the thing acted upon is the subject, then it's passive:
The ball was thrown by Euclid.
Here the ball is the subject of the sentence, and since it's what the action happened to, we're in passive voice.
I think the point about the "copular" thing is that a common sign of a passive construction is the use of "was" (or "were", etc), but that just seeing "was" doesn't make a sentence passive:
The ball was thrown: passive
The ball was red: active
It's not even just "was" with a verb form:
I was riding my bike: active.