To call before. Any time you seek to have an entity come to you, whether it is in the astral or the physical you are evoking. Going where you think the entity is and hoping it shows up is an invitation and is therefore evocation.
to call within. Most sensibly done with the godhead of your faith but many folks practice invocation of lesser beings. Channeling, automatic writing, speaking in tongues, possession are some examples.
Invocation of an entity. This may be complete or partial. Even with full on nasty demonic possession the person isn't walking around "possessed" 24/7. There are gaps.
All possession is consensual but some cases are inadvertent. This is most often due to ignorance. In demonic possession cases it is usually just due to the subject being worn down by the influence of the entity (giving up or in to it). You'd be amazed at the number of New Age types who decide to "channel" something and don't realize what that means.