This section is from the book "Some Contributions Of South India To Indian Culture", by S. Krishnaswami Aiyangar. Also available from Amazon: Some Contributions of South India to Indian Culture.
The literature pertaining to this school falls into two classes also, similar in character to that of the Saivas of the Siddhanta school. The Vaishnavas have their saints and their teachers. They count twelve of the former and give them the general name Alvars. The latter go by the name Acharyas, and constitute a continuing series differing for each group and in some cases even for smaller groups of families, as the list proceeds to nearer modern generations. The twelve Alvars composed hymns in praise of Vishnu in the various forms in which his representation is worshipped in the various temples of South India. These partake generally of the character of the pauranic pantheon of the Hindus. The most popular of these, of course, are Rama and Krishna; but references are traceable in their works to the most recondite manifestations of Vishnu referable to the Pura-nas. The twelve Alvars fall into three groups : the early, the middle and the later ones. The first of them consists of four names, of whom three are undoubtedly contemporaries and the fourth is certainly so regarded by the Vaisnavas.
These four are Poygaialvar, Bhudattalvar, Peyalvar and Tirumalisaialvar. In the middle group come Nam Alvar and Madhuraakavi, followed rather closely by Kulasekhara, Periyalvar or Vishnuchitta and the daughter of the last, Goda or Andal. The last group is composed of Bhaktangrirenu or Tondara-dippodi, Yogivaha or Tirup-Pan Alvar and Tirumangaialvar, the last of them all. The works of these Alvars and their poems of devotion are admittedly renderings of the teachings of the Veda and Vedic literature to a far greater degree than even the literature of the Saiva Adiyars. On a historical examination of this orthodox order of the Alvars and their grouping according to chronology, it has been pointed out elsewhere1 that there is sufficient historical evidence available to make the accepted order more or less correct, and that the range of time during which this particular group flourished corresponds to the age of the Pallavas, as in fact the age of the Saiva Adiyars was already pointed out to be. If the age of the Pallavas was characterised by the infusion of northern culture into the distant land of the south, this provides an excellent and unmistakable illustration of the position.
1 See my Early History of Vaishnavism in South India (Oxford University Press).