A cosmology is a group of planes related to a specific Prime+Material world or mythos, or a particular view of the Multiverse. The sages of Toril currently see the multiverse as a branching tree, the worshippers of the Greek or Olympian pantheon see it as a mountain, and the worshippers of the Norse or Asgardian pantheon see it as a tree between poles of fire and ice. The people of Oerth use the cosmology of the Great Wheel.
Some speculate that there is but one multiverse and that each individual culture describes it in a way that works best for them, unconsciously picking and choosing those planes they are familiar with and ignoring the rest. Others insist the only way to travel between cosmologies is the Plane+of+Shadow.