chemistry - electrophilic additionWatch
In general EWG and EDG depends on the effects, chemists make differents for two: the positive inductive (+I) or mesomeric (+M) and the negative inductive (-I) or mesomeric (-M) effect. According to what kind of effects you have electrons are donating or withdrawing in a conjugated pi system. This conjugated system is nothing more than a bond with alternating single or double bond in a aromatic compound.
When you have a positive mesomeric or inductive effect, the electrons get attracted by the activating group. In this case it is an electron donating group.
When it is a negative inductive or mesomeric effect, the electrons get removed by the activating group. That is an electron withdrawing group then.
The link below show you all kinds of groups for both effects: