Ahh, Counterspelling.. definately one of the coolest features from 3rd edition
Certainly my favorite ability hehehe.
Basically, counterspelling is devoting all of your attention to a single enemy spellcaster, and try to hinder him to get any spells cast at all.
What you do manually: Rightclick the enemy mage, choose counterspell, and watch. Or, assign a hotkey to counterspell like me.
What happens then: First of all, you must have some reflexes, or the enemy will get his spell cast, before you even notice. Then, most important, you need a high spellcraft skill, in order to recognize the name of the spell he is casting.
If all is ok until now, the counterspelling fun can begin!
The enemy casts lets say magic missile.. if you have a magic missile memorized, too, you will counter him. That means, the animations plays, but his magic missile will not be cast. You will see a message above his head, that you countered him. Both will loose their memorized magic missile for that day.
You can counter every spell:
a) If you have exactly the same spell memorized.
b) If you have one of the dispel magic spells memorized. (Lesser dispel magic, dispel magic, greater dispel magic, and mordekainen's disjuntion) The dispel magic spell-level must be at least the same level, and it will always use the lowest possible version to counter. Bad luck is, if you have only greater dispel left (level 6), but your enemy casts magic missile (lvl 1), because that would be a big waste of your spell.
c) If you have one of the additional counterspells memorized, which are listed in the spell descriptions.
For example: If the enemy tries to cast haste on himself, you need to have either the haste spell memorized yourself, or one of the following: dispel, greater dispel or mordekainen's (lesser dispel doesn't work because it has level 2, while haste is level 3),
OR the slow spell memorized, because the spelldescription of haste says: "Addictional counterspells: slow".
Btw, sorcerer's are absolutely the best counterspellers, think about this
Boney