Leca [Light Expanded Clay Aggregate] clay pellets or clay balls are Marketed under several trade names. Geolite, Grorox, Hydroton are a few brands, the process and product varies ever so slightly from brand to brand, but they are all comparable.
LECA is very popular as a Hydroponic Growing Medium in all system types. They provide an ideal balance of moisture, food and air. Root rot issues are minified by the space between and within the pebbles allowing all-important oxygen to get to the plant roots.
Porosity - oxygenation
Inert - pH neutral - zero nutrient value
Drainage - are sometimes added to denser media to help promote drainage.
LECA does not retain much water when used by itself, it needs to be frequently re-watered. LECA Clay Ball are sometimes used in conjunction with other growing media to improve oxygenation.
Reusable to a certain extent but Expensive. After a harvest the rocks need to be washed to remove old roots and traces of minerals. Sterilizing with white vinegar, diluted food grade peroxide or extremely diluted bleach at a ratio of 1 part bleach to 10 parts water.
LECA doesn't retain moisture as well as many other substrates, this is another reason it is frequently combined with another medium.