Hi @Naxas,
Strength attribute will increase the damage of the physical hits, while Magic will increase the damage of the spells. If you hover over the attributes on the team screen, you can see a description for each statistic.
I hope that helps!

so, my summoned skelletons will need (my) strength to hit harder?
or are they compleetly unaffected by my own Attributes?

The skeleton's stats are affected only by your intelligence. So more intelligence means your skeletons will be higher level.