Step 1: One-line identity for each: Alvars = Tamil Vishnu-devotee saints; Nayanars = Tamil Shiva-devotee saints — both active roughly between the 6th and 9th centuries CE.
Step 2: What made them distinctive: Unlike Sanskrit-based Brahmanical ritual, they composed and sang their devotion in Tamil, the language of ordinary people, and drew followers across caste lines, including women (e.g. Andal among the Alvars).
Step 3: Their legacy in texts: Alvar hymns were compiled into the Nalayira Divya Prabandham; Nayanar hymns became the Tevaram and Tiruvachakam, both still recited in Tamil Shaiva and Vaishnava temples today.
Step 4: Historical importance: Their emotionally intense, vernacular devotionalism is regarded as the starting point of the broader Indian Bhakti movement that later spread north through figures like Ramanuja, Kabir, and Chaitanya.
Final Answer: Alvars (Vishnu) and Nayanars (Shiva) were Tamil bhakti poet-saints whose vernacular hymns, later canonized as sacred texts, founded the Bhakti tradition and made devotion accessible beyond caste and Sanskrit learning.