Perlite is a volcanic rock that provides more airy substrate and more thorough drainage. You can use perlite to improve the structure of the potting soil and to improve its water retention capacity.
Hydro granules are small spheres made from clay that improve the drainage of plants. A layer of hydro grains under the potting soil can prevent root rot. In addition, hydro granules are a cheap and light option to fill up your pots.