A stable Garganta, created by an Arrancar, appears like a mouth opening up in the sky. Less stable Garganta, which are more common and are opened by lesser Hollows, appear more like rips, folds, or tears in the sky. Garganta can vary in size based upon the size of the Hollow or Arrancar coming through.
Garganta are turbulent spiritual pathways in-between worlds. A turbulence of Reishi blows through them and those that seek to use them as passage must gather and condense this Reishi below their feet to move. The better one's control over their spiritual energy is, the cleaner the path will be. For instance, Ichigo Kurosaki's paths are narrow and sloppy due to his poor control over his energy.
Garganta can be opened by Hollows, Arrancar, and Shinigami through Kūmon, Descorrer, and Kidō respectively. These portals can also be open by special equipment, such as the machinery discovered by Mayuri Kurotsuchi in Szayelaporro Grantz's laboratory.