In the Corey's answers and errata pdf, "turn" is mentioned quite often, like so:

Q: Can a hero use the Staff of Light Reward card to stand
up his controller's routed units during the middle of a battle?
In other words, are these units considered to be in the area
the hero is in?
A: Yes, the hero can use the Staff of Light mid-battle as
long as it is during his controller's turn. This would function
much like the Rally Cry Tactics card.

Now, I always thought that a "turn" was "when you are performing the actions pertaining to your order card". This would mean that things like the Staff of Light (and Tobin Farslayer's ability) would only work for the attacker? (Since the defender would not be fulfilling HIS order card).

Am I correct in this? Or is there also a "battle turn" or some such, when your units are performing their actions?

You are correct - a "turn" is when a player is executing his order (as well as immediately prior or following that). Thus, as you point out, only the attacker could use the Staff of Light (because it says "during your turn"). However, the question you refer to doesn't contradict this - it does specifically say it can only be used during the turn of the hero's controller. So if the hero belonged to the defending player, he could not use it, as it's not the hero's controller's turn.

However, it should be pointed out that while the question does address the "only on your turn" bit, the main point of that question was to emphasize that the units who are participating in the battle are still considered to be "in the area" for such purposes (even though they are physically off the board an near the player's race sheets).

EDIT - You mention "Tobin Farslayers" ability, though; Tobin's ability doesn't require it to be that player's turn to use the ability. If the defender controls Tobin and he is up to 1 area away from the battle, he could use the ability in the battle. Same with Mad Carthos. Basically, the only time the effect would be limited to the attacker is if it says "during your turn".

Ahh, thank you for clarifying that