can someone explain what the spacing guild is in dune?

Question

why do only some of them look unhuman?

Answer 1

The "navigators" are non-human looking because of the great amount of spice they take. They have evolved and have the ability to manipulate space so that ships can travel great distances. The rest of the guild are employees of the navigators and are human. I think they have some sort of mind link with the navigators. They are the only ones allowed to see a navigator.

Answer 2

Some of the Spacing Guild are changed by massive use of spice, others are their employees that do not take that much spice.