The practical tradition distinguishes two operations, and the distinction structures everything in the grimoires.
Invocation calls a spirit inward, the operator seeks communion with, or the presence of, a higher intelligence, classically the holy guardian angel, whose attainment is the central aim of the Abramelin operation and of much later ceremonial practice. Evocation calls a spirit outward, to visible or perceptible appearance in a triangle or vessel outside the protective circle, and constrains it to a task. The apparatus that follows, purification and fasting, timing by planetary hour and day, consecrated tools, the drawn circle, the seal or sigil of the spirit, the names of God used as constraint, is the machinery of the second operation.
The classic texts are the Key of Solomon, the Lesser Key with its Ars Goetia, the Heptameron, Agrippa's Three Books of Occult Philosophy with the spurious Fourth Book, and the Abramelin material.
Two source-critical points belong here. First, the demon catalogues were published by a sceptic: Johann Weyer's Pseudomonarchia Daemonum of 1563 lists sixty-nine spirits and their offices as part of an argument that the whole business was delusion and that accused witches should not be executed. The Goetia's list descends from Weyer's. The tradition's standard reference work is therefore a debunking text that was subsequently used as a manual.
Second, some entries are demonstrably invented. Enochian, the angelic language and system received by John Dee and Edward Kelley in the 1580s, is a separate and internally elaborate system whose status is disputed even among practitioners, and Dee himself recorded doubts about Kelley.
Practitioners describe results in terms ranging from literal appearance to psychological operation on the operator, and that disagreement about what is being contacted is as old as the practice.