First some definitions:

Invocation - 1. the act of invoking or calling upon a deity, spirit, etc., for aid, protection, inspiration, or the like; supplication. 2. any petitioning or supplication for help or aid. 3. a form of prayer invoking God's presence, esp. one said at the beginning of a religious service or public ceremony. 4. an entreaty for aid and guidance from a Muse, deity, etc., at the beginning of an epic or epic-like poem. 5. the act of calling upon a spirit by incantation. 6.the magic formula used to conjure up a spirit; incantation.

Evocation - 1. The act of calling out or forth. 2. Creation anew through the power of the memory or imagination. 3. from evocare "call out, rouse, summon," from ex- "out" + vocare "to call". Evoke is more often or less with a sense of "calling spirits," or being called by them. 4. Evocation was used of the Roman custom of petitioning the gods of an enemy city to abandon it and come to Rome; 5. it was also used to translate the Platonic Gk. anamnesis "a calling up of knowledge acquired in a previous state of existence."

 

Many folks use these terms interchangeably but they are really quite different in practice. Evocation is a much more intimate act than invocation.

 

Being 'ridden' by a spirit in the Voudon tradition is an example of an evocation. You are evoking the spirit into yourself.

 

Bringing up a spirit within a formal ritual circle and tradition is an example of an invocation. You are invoking the spirit to appear before you.

 

Some questions for you to consider...
1. When is invocation more effective?
2. When is evocation more effective?
3. Are they really separate phenomena or do both flow from a common well?
4. What circumstances promote more effective invocation or evocation?
5. What are your personal experiences with invocation and/or evocation?