The rules say that counterstrike allows a defending unit to immediately damage an attacker whenever it is declared as a defender.

Is this saying that counterstrike does its damage,possibly killing weaker units before they can do their damage? Also,is all counterstrike the same? Some cards say in parentheses(this unit immediately deals 2 dmg. when defending),while others say(this unit deals 4 combat dmg. immediately AFTER defending).

This confused my friend and I the other day while playing. I thought all counterstrike was the exact same,except for damage of course.

Yes, counterstrike damage is dealt as soon as the unit is declared as a defender, so it can kill an attacker before he gets to deal damage.

As for the wording, that just depends on the when the card was printed. As the FAQ says: All instances of "after" in card text boxes should read "when".

Also from the FAQ:

Counterstrike
Counterstrike is a keyword that allows a
defending unit to immediately damage
an attacker whenever it is declared as
a defender. Counterstrike damage is
combat damage that cannot be split
among attackers, is uncancellable, and
is applied as soon as it is assigned,
before any other actions can be taken.

I love counterstrike units.

Thanks for clearing that up for us.