In a magickal context, the definitions are as such:
'Evocation' : The act of evoking : summoning - as in 'the summoning of a spirit'.
Evocation means to evoke, to manifest something, figuratively or literally. it could be done by summoning (external) or by channelling (internal). You can also evoke an emotion in others though, the term is not limited to spiritualism. Evoke is very synonymous with 'inspire'.
When an exorcist says something like "the power of Christ compels you", like in Hollywood, that would be Invocation - invoking inferred authority. When a spiritualist in a seance says "we invite the spirit of this house to come to us"; that would be evocation. When you tell a joke to a friend to make them laugh, evocation. When you insist someone move their car out of the no parking zone, invocation.'Invocation' : The act or process of petitioning for help or support; a humble prayer, usually performed at the beginning of a service of worship. Invocation can also mean 'calling upon an authority (supernatural force, in this instance) for justification'.
Invocation means 'to invoke', which is a slightly less out of date word. You can invoke a clause of a contract, for example. it means to insist upon something by way of inferred authority. It might be used in summoning or channelling, if your method is 'forceful' or supported by pre-established rules or agreements, rather than being congenial.
If you are performing an invocation, you are not expecting the spirit to turn up, you are instead requesting assistance. If you are performing an evocation, then you are wanting the spirit to manifest for you. An invoked entity could still manifest, but would do so as a result of it deciding to do so itself - an evoked entity must manifest in some form, otherwise it's simply been invoked.
In a purely technical sense, the two words mean the same thing. The whole evocation means this and invocation means that is relatively new, and was popularised by Crowley. In some magical texts there is little distinction between the act of "prayer" and the act of calling a spirit - you are in a sense doing the same thing.
Hope this clears things up, if you were confused [grin]