If we want to define Siddhas in a word, they are "marmayogis", in possession of the great secret of living, not easily understood by ordinary men. They have understood the meaning of the cycle of birth and death. They have directly experienced the divine by means of their meditative powers. But despite their detachment, they are full of love and goodwill for ordinary men.
Siddhas have taken human birth only to hold up a mirror for the self-development of ordinary individuals. They have shown the infinite treasure house of self knowledge.
The siddha system of medicine is synonymous with the Tamil system. The mushrooming of yoga centers all over the country owes its genesis to their system of medicine.
They have performed several wondrous deeds. For example, walking on water and fire, flying in air, leaving one body and assuming another, transforming ordinary earth to gold, these are some of the things they have shown. Whole mountains have been transformed into gold.
A siddha saint who lived the life of an ordinary goatherd shook the nine planets. It was a siddha saint who helped Raja Raja Chozhan to install a lingam in the Thanjavur temple.
In Srirangam, the presiding deity happily gifted his jewels and another siddha saint controlled Kali in a cemetery. Yet another siddha saint dug a secret underground passage to Palani in order to clarify his doubts.
Siddha saints have worked many great deeds. Listen to this audio explanation of the greatness of Siddha saints. You too will be electrified by it.