I found really good definitIon for your question from the pdfs i am reading recently.
" Evocation can be defined as the calling forth of an entity from another
plane of existence to an external manifestation in either the astral or physical
plane. Evoked beings are brought closer to the magician, but never within
himself or herself. This is what separates evocations from invocations. In an
invocation, the magician brings some foreign intelligence within himself or
herself, and allows the entity to speak through his or her body. Channeling is
a well-known form of invocation.
In an evocation, however, the magician brings the entity to a plane where
the magician can view it and communicate with it. Evocation is therefore an
external manifestation of an entity, as it occurs outside of the magician's body"