Step 1: Define the two groups: The Alvars and Nayanars were groups of Tamil poet-saints (roughly 6th–9th century CE) who were the earliest wave of the Bhakti movement in India, spreading intense personal devotion in the Tamil-speaking south.
Step 2: Alvars: The Alvars (meaning 'those immersed in devotion') were devotees of Vishnu; there were traditionally twelve Alvars, including Andal (the only woman among them) and Nammalvar, whose hymns were later compiled into the Divya Prabandham.
Step 3: Nayanars: The Nayanars were devotees of Shiva; traditionally counted as sixty-three, including figures like Appar, Sambandar, Sundarar, and Manikkavachakar, whose hymns form the Tevaram and Tiruvachakam.
Step 4: Their significance: Both groups composed devotional poetry in Tamil (not Sanskrit), making bhakti accessible to common people regardless of caste or gender; they travelled between temple towns singing hymns, directly challenging Brahmanical ritual exclusivity, and their compositions were later canonized as sacred texts recited in Tamil temples — laying the foundation for the wider pan-Indian Bhakti movement that followed.
Final Answer: The Alvars (Vishnu bhaktas) and Nayanars (Shiva bhaktas) were Tamil poet-saints who popularized vernacular, caste-inclusive devotional worship, founding the Bhakti tradition later canonized in the Divya Prabandham and Tevaram/Tiruvachakam.