Traveling between Material Planes
There are two ways to move from one Material Plane to another: Portals, which are found naturally and can be created artificially, and Planar Shifting, where one teleports from one Material Plane to another.
Portals
Portals, natural or artificial, have two sides, each one on a separate Material Plane. However, one side of a Portal may be in a fixed position on one Plane but mobile on the other. For instance, all naturally occurring Portals on the Transitory Plane of Earth are in a fixed location but all Portals, natural or artificial, on the Cosmic Plane are mobile. This is because the sides of the Portals on the Cosmic Plane are all on planetary bodies which rotate on an axis while orbiting a Star. Artificial Portals, which are always part of a Planar Gate, can be mobile on any Material Plane. (Indeed, that mobility can be the reason they are created.) Regardless of their origin and mobility, Portals are the primary method of traveling between Material Planes.
Planar Shifting
Planar Shifting is when a person uses Arcane or Psionic (or sometimes Divine) Magic to move from one Plane to another. This is essentially a type of Teleportation that exploits the Resonances of the different Material Planes. Each Plane has its own Resonance. When one moves through a Portal to another Plane, one's Resonance changes to match the Resonance of that Universe. However, if one changes one's Resonance to match the Resonance of a different Plane, one will shift to that Plane. This is easier said than done, of course, but it is further complicated because one must also be able to control where one materializes on the other Plane. One could very easily miscalculate and end up in a location on that Plane one did not wish to visit. This is why Planar Shifting is not a very common method of moving between Material Planes. It's simply easier and safer to rely on Portals.