From Ardrana

Invocation and evocation spells channel magical energy to create specific effects and materials. Invocation normally relies on the intervention of some higher agency (to whom the spell is addressed), while evocation enables the caster to directly shape the energy.

Note: Invocation as described here originally appeared in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.