Attacks can have a number of Effects, although each attack can only have one, except for the two basic effects.
The two basic effects are MAP and Counter. These are the only effects that can be combined with others (but not each other). However, combining MAP with anything else is exceedingly rare, Counter slightly less so but still unusual. These two effects can also be temporarily added with Spirit Commands.
Counter attacks can be used in the Counter reaction. Attacks without Counter cannot. Assail temporarily adds this effect. Counter is the most common effect and almost everybody has two or three, usually on 'basic' attacks.
MAP attacks can be used to target multiple people. Attacks without MAP cannot. Zeal temporarily adds this effect. While not as common as Counter, MAP attacks are not unusual and most people have one or maybe two.
The other effects cannot be added by any Spirit Command; they are either on the weapon, or they are not. No attack can have more than one of these effects. Generally, attacks with these effects have a higher Will requirement, take EN or ammo, are not very powerful damage-wise, or all of the above. None of the effects last for more than one round and some are instantaneous.
The attack must do damage to add the effect. Further, there is a chance that the effect will not work (although damage from the attack will be processed normally). Counter and MAP attacks always work as Counter and MAPs.
Weaken attacks reduce the damage dealt by the target. Degrade attacks increase the damage to the target. Jam attacks reduce the accuracy of the target. Slow attacks increase the hit rate against the target.
Drain attacks reduce the EN of the target. This effect is quite rare. Enervate attacks reduce the Will of the target. This effect is exceedingly rare; don't expect to have it without a very good reason. Enervate attacks are often Magic.