Visions were the true form prophecy took in the mind of a prophet. After the vision, the prophet would write down what he saw, but the words were only meant for another prophet, who would see the original vision after reading the prophecy. Sorceresses and wizards without the gift of prophecy may have been able to have a limited understanding of the prophecy.
Seers also had the ability to see visions of the future, but these visions were not the same or as strong as a prophet's. In both cases, visions were spontaneous.
Visions could be broken into true and false forks. Only a prophet with enough ability could differentiate between the two.