Sri Rudram is a Vedic prayer devoted to Lord Shiva in one of his several forms, which is Rudra form. Divinity manifests itself in millions and millions of forms, and in the ultimate view, the entire Creation is considered to be a manifestation of Divinity (known as Vishwa Roopa Eashwara). These millions and millions of forms are condensed into 33 main Devathas, of which 11 Rudras are an important constellation.
There are different lists of the names for these eleven Rudras: In the Hari Vamsha, these are listed as Hara, Bahiroopa, Thryambaka, Aparaajitha, Vrushaakapi, Shambhu, Kapardhee, Raiwatha, Mrughvyaadha, Sharva & Kapaalee. The Shambhu is also known as Shankara. The Vishnu Puraana gives a different list that consists of Manyu, Manu, Mahmasa, Mahan, Siva, Rtudhvaja, Ugraretas, Bhava, Kama, Vamadeva and Dhrtavrata. The Brhadaaranyaka Upanishad further tells that these eleven Rudras are symbolic of the ten sense organs and the mind (the ten sense organs consisting of the five organs of perception – Gnyaanendriya – consisting of the eyes, nose, ears, tongue and the organ of touch (skin) and the five organs of action consisting of the hands, legs, speech, organ of excretion and the organ of reproduction)
However, the Sri Rudram (also known as Namakam) itself describes Shiva in terms of everything seen and experienced in this creation and is therefore considered a description of the Vishwa Roopa Eashwara, wherein, the One Divinity expresses itself as EVERYTHING in this Creation, thereby teaching us to view this entire world as Sacred.
The Sri Rudram consists of eleven Anuvaakas or Sections and is traditionally chanted in different formats. At the simplest level, Sri Rudram is chanted once and then is followed by Chamakam, which is another Vedic Prayer that seeks blessings and boons from Lord Rudra, who is none other than Lord Shiva. Starting from this simplest format, the next higher formats expand as follows: Sri Rudram is chanted eleven times and then is followed by a one-time chanting of Chamakam – known as Rudra Ekaadashini; Sri Rudram is chanted one hundred and twenty one times (121) and then is followed by a eleven times chanting of Chamakam – known as Athi Rudram; Sri Rudram is chanted one thousand three hundred and thirty one times (1331) and then is followed by one hundred and twenty one times (121) chanting of Chamakam – known as Mahaa Rudram.
It is customary to chant Sri Rudram during Pradosham Times and also during Mahaa Shivarathri. There are people who observe Shivarathri Vratham on a monthly basis (the night that falls before the new moon day), and Sri Rudram chantings on those monthly Shiva Raathris is also seen. In temples, particularly in Shiva temples, Sri Rudram is chanted during the Abhishekam performed for the Lord Shiva. Traditional and orthodox Devotees who worship Lord Shiva in their homes, in the form of Saaligrama or Shiva Linga also chant Sri Rudram during their daily prayers.