In conventional seasoning, the oil or fat is converted into a hard surface at or above the high temperatures used for cooking, analogous to the reaction of drying oils. When oils or fats are heated, multiple degradation reactions occur, including decomposition, autoxidation, thermal oxidation, polymerization, and cyclization.
Often cookware's seasoning is uneven, and over time it will spread to the whole item.  Heating the cookware (such as in a hot oven or on a stovetop) facilitates the oxidation of the iron; the fats and/or oils protect the metal from contact with the air during the reaction, which would otherwise cause rust to form. Some cast iron users advocate heating the cookware slightly before applying the fat or oil to ensure it is completely dry.
The seasoned surface is hydrophobic and highly attractive to oils and fats used for cooking (oleophilic). These form a layer that prevents foods, which typically contain water, from touching and cooking on to the hydrophilic metallic cooking surface underneath.
The seasoned surface will deteriorate at the temperature where the coating breaks down. This is typically higher than the smoke point of the original oils and fats used to season the cookware. Thus old seasoning can be removed at a sufficiently high temperature (~500 °C), as found in oven self-cleaning cycles.