Good news for those with short attention spans - this part isn't very long at all.
This is what the third type of card in Arrows looks like. Unlike Field and Effect cards, these are not put down onto the field. They can be used at any time, including during the opponent's turn, as long as the card meets the requirements that are listed on every card of this type. They also don't have any cost to them, so you don't need to earn any points to play them.
Using Special cards allows you to cause problems directly to your opponent instead of having to do it from the field itself. They can also be used to catch up and even up the game - several Special cards have requirements that your opponent must have more points than you, or more Arrow pieces than you. They can even counter certain situations, like when your opponent plays a useful card or uses a very strong attack.
These, combined with the other two types of cards, can cover just about everything the game needs to make things interesting, especially since Special cards can do almost anything. Off the top of my head, they can interrupt an enemy attack, destroy certain Field or Effect cards, clear the whole field, change your score to match your opponent's, lower your opponent's score, let you instantly gain points, swap the positions of enemy cards or your cards, rotate all Effect cards, and there's plenty more I can think of eventually.
Not much else to say here; these ones are pretty simple. It took me a while to learn this, but simple is good. The next thing I write here will cover my previous, pretty complicated card game, and everything that was horribly wrong about it.