/invoke is for area effect spells.
/cast is for long range spells.
Invoke works right away for spells like icestorm and burning hands. "/invoke burning hands"
Cast is for small fireball and lightning...long range spells. I think you need a staff to cast these spells. They are expensive and we can't buy them yet. I think they will add a starter staff later, so be patient.
Hope that helps you.
