Fireball is much like the arrows card but does more damage to a smaller area. Once you have figured out that they have a fireball you can bait it out by sending some smaller troops and trying not to bunch your troops together too much.
It is very popular to use on a group of barbs, if the barbs are played at the back of the field it will be used to cause damage to the tower and the barbs, and if they are played aggressively then they are usually targeted on the bridge leaving the barbs with only a small amount of health left and by the time they reach the tower they are basically all gone, depending on the level of the fireball vs the barbs.