This definition from Wise Geek explains it fairly succinctly:
Invocation and evocation are English words that are often used interchangeably. They are both derived from the Latin word vocare, which means to call forth. Both words can deal with summoning interaction with non-human entities. Yet, many people, especially those with knowledge of the occult, believe there are differences. These are generally based on how an entity is summoned and which entities are being dealt with.
The first notable difference between evocation and invocation involves the distance between the entity that is being called forth and the person who is doing the calling. With an evocation, a non-human entity is summoned, but is not linked to the caller. The entity remains in an outer area. The energy that is used to summon the entity is also believed to be derived from outside of the caller.
With an invocation, the caller becomes a medium. The entity that she summons is meant to come forth within her. This person is generally believed to use inner resources or energies to allow this to happen.
Image: "Ghost of Samuel appearing to Saul" by William Blake